In most cases, yes. But you'll also typically find the model number on the info panel of the device (usually on the bottom). Such as the following image:
Routers often have multiple MAC addresses -- sometimes a different one for the WAN and LAN, and then also separate addresses for the wireless hardware.
An ER-X actually has 6 MAC addresses and it doesn't even have wifi built in. Basically it is one for each port (if the ports are used individually) + 1 for the switch (the ports can be assigned to the switch or used independently). They are all sequential in the case of the ER-X. And likewise, you're seeing an address that differs only by 2, so there's probably at least one other MAC address in the system that you're not aware of that occupies the b7 space.
So depending on how the ports are assigned (or how you determined the MAC in the first place), it is plausible for the MAC address to change.
I still don't know what specific hardware you are using, so I can't comment with any certainty. But 21.02 is larger than 19.07, so yeah, totally possible to run out of space. EDIT: I see that you did change your title to indicate that you are using at TL-WDR4300 -- this is an 8MB device, so it makes sense that you could run out of space. I answered in your other thread that you can use extroot.