But we are Talking about MX65......
As I said, I tested on an MX65W this morning and it was not powered from POE. YMMV.
I cant find my 48 Volt AC adapter to show you (have LAN 48V Injection) how it boots from POE.Have three MX65's One has OpenWRT other two stock firmware (one is wi-fi version).
Feel free to post your setup.
Here is mine, no power to the MX65W:
The same 802.3at injector works fine for POE powered devices like MR33, MR30H, MR36, etc.
Try a 802.3at/bt (POE+/++) injector.
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My MX65 also does not take power from POE (my POE switch supplies 60w). When its is powered with a DC 54V then it does power up the two Meraki MR52 from the port 11 & 12.
I tried to see if my MX65 with Original CISCO Meraki firmware will power up on 60w POE supply (but not a 802.3at) from switch and it would not power up.
I supplied 12V, 1A DC (12W) and no POE, it powered up and was functioning. At this voltage and at 24V DC, it did not provide POE to downstream devices on port 11 and port 12.
When I supplied 54V DC, it not only booted up, it also booted two downstream devices.
Well, that's not surprising since no firmware is running at the moment device gets supplied with power.
As @kitor stated, do not ever expect PoE input to work reliably. This is only a quirk and not a documented feature.
And do not ever expect PoE outputs to work at all when the device is starved from proper supply voltage.
You're running the device out of specification, do not assume ANYTHING.
I think, You don't understand how POE is working.
POE at/af is not passive POE.
After you connect two devices (one ex. Switch with POE, and the second ex. AP with POE - both compatibile with ex. POE 802.11at).
First the switch is detecting is the connected device compatybile with the standards 802.11at if yes, the switch is upper the output voltage (on the specific pairs of patchcord) if not, the voltage is the same as normal devices like PC or someting else without POE.
Passive POE have alwayes on the specific pairs of patchcord the specific voltage. If you have POE Injector 54V, You have 54V output on specific pairs, if 36V - 36V on patchcord.
Normally the Passive is not compatibile with POE at/af (POE/POE+)
Because MX65 is not compatibile with POE input, so you cannot supply this device with POE - only Power Adapter, but The MX65 can supply another device because has POE output (not POE input).
you can read about PoE and PoE discovering.
POE is not only a software 
There may be some logic to it. I will try to find a switch that has 802.3at or 802.3bt on its POE port. I can report my test results after that.
This may be the key.
How do you want to supply the mx65? Just connecting patchord to one of the ports?
That doesn't work, because the MX65 is not compatibile with POE supply. You van supply it only with AC adapter.
You can "take" from MX power to power another device compatibile with POE+ (if I remember correctly)
First problem is that the mx65 should have the resistor on the port "Poe In" to another divice can discover it is compatibile with POE. Second, from this port it should have a POE converter - I think a transformer...
I doubt it has both...
If you can only supply it with the AC adapter, how come theres proven evidence that it can be powered via POE?
I agree that MX65 is not expected to supply POE power to downstream devices if a non standard power supply is used. There are pictures showing that could be powered using a POE and so that leaves us with one possibility:
Some MX65 would power up and some wont if a correct source of POE (802.3at/bt) is attached. This is possible between rev numbers but I am skeptical on this. I think the key is proper POE supply.
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Meybe it is a little good luck, but I think not standard work.
I'm waiting for Yours tests and answers. Good luck.
Because detection mechanism used by some PSEs are lax enough, to see almost any loads as "proper". And reverse conduction of protection diodes in the built-in PSE might make the device power up, by backfeeding the DC port. Which, worst case, may cause PSE part to overheat, because the protection diodes should not normally conduct at all.
FYI, even a passive 48VDC injector won't power my units on. One that forcibly puts out 48V without any detection logic.
Could I ask moderator to split this topic out of the image support? Because we're going way OT here from post 429 onwards.
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@Leo-PL that is good explanation, and how I said, not standard work.
e.g. Edgerouter -X has one port marked by the manufacturer as POE-IN and can be powered by another device via a patch cord. It work as manufacturer project and want it.
In MX65 case POE -IN it work as manufacurer doesn't project it and doesn't want, and you can just damage your device.
Oh, trust me, I know how POE works, I'm a network admin
@Leo-PL did answers for your question from electronic side...
This proof doesn't mean, that the device should be supply by POE. This is only the bug on electronic side and not wanted work by manufacturer.
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Which might explain why I read some where on the Meraki official forum that went something like this:
While some of the MX65 devices may power up using POE as input, it is not a supported use.