Ok, reverted back to xiaomi stock (to test the roaming). The roaming is working flawlessly... Im not sure what kind of roaming they are using. In the wifi scanner app I have, I see:
[WPA2-PSK-CCMP][RSN-PSK+SAE-CCMP][ESS]
BTW-when roaming, its on the same channel...weird.
Disappointed with the openwrt roaming 
To be clear, OpenWrt doesn't have anything to do with roaming. Unless you are running 802.11r/k/v, the AP actually has literally nothing to do with it, ever, regardless of the firmware on the AP. It is a client side process.
So, if your results with OpenWrt are not as good as the stock firmware, that suggests one of two things:
This is generally a bad idea for roaming. Roaming works best when the channels are different.
Noticed extra hidden SSID.
I mean 2nd AP is broadcasting the Wifi SSID and one more Hidden SSID , Intersting...
Not sure what you mean by this... maybe you can clarify which AP was broadcasting a hidden SSID and if it was the same SSID name as your main network or something else.
the patch is not in stable version yet.
Are we talking about the same solution?
Seems like something different..
it is about patch that resolves lack of roaming/FT in versions 21.* and 22.*
I am also experiencing the same issue on AX3600 running Openwrt wherein the client can't connect to the nearby router even when i am sitting right next to the router. this is super odd
I tried with the stock firmware, roaming works very well.
When standing next to my 2nd AP it immediately connected. (No 802.11r/k/v, checked the wifi name with app)
With openwrt, its not roaming..
I tried to disable "Disassociate on low acknoledge" still no luck..
What could be the reason?
You mentioned you checked the network name, so you have different SSID's?
Try using the same SSID but just different channels.
Im using the same SSID name & pass & security
Here:
How do you revert from OpenWRT to Stock firmware? Can you share the steps please?
And that script that I referred to earlier wasn't helping after observing for a long time. Its just a brute force and I removed it from startup. I guess at this point we have to tune 802.11r/k/v.
To check if 802.11r/k/v is applied, you can run software like WinFi or WiFi Explorer Pro and check 802.11 amendments
Is this command required? uci set wireless.default_radio0.ieee80211v='1'
I noticed my 802.11v is enabled even without that.
Update: Confirmed that it's no longer required from this commit
My partial radio config /etc/config/wireless
Things to note:
mobility_domain should be same across all APs. I have two SSIDs - one for 2.4 Ghz & another for 5 Ghz. Hence I use two different mobility domains for different radios.
alter option time_zone accordingly.
config wifi-iface 'default_radio1'
option device 'radio1'
option network 'lan'
option mode 'ap'
option ssid 'Who5'
option key 'removed'
option ieee80211r '1'
option mobility_domain 'b00f'
option ft_over_ds '0'
option ft_psk_generate_local '1'
option bss_transition '1'
option wnm_sleep_mode '1'
option time_advertisement '2'
option ieee80211k '1'
option reassociation_deadline '20000'
option time_zone 'GMT0'
option encryption 'psk2+ccmp'
config wifi-iface 'default_radio2'
option device 'radio2'
option network 'lan'
option mode 'ap'
option ssid 'Who'
option key 'removed'
option ieee80211r '1'
option mobility_domain 'beef'
option ft_over_ds '0'
option ft_psk_generate_local '1'
option bss_transition '1'
option wnm_sleep_mode '1'
option time_advertisement '2'
option ieee80211k '1'
option encryption 'psk2+ccmp'
option reassociation_deadline '20000'
option time_zone 'GMT0'
I just used official tool from xiaomi: MiWifiRepairTool.x86
I enabled r/k/v , but I see only [FT] in my android app...
Why I dont see [K] and [V] ?
Check using WiFi Explorer Pro 3 for Mac or WinFi For some folks, WinFi doesn't work. So YMMV. It should be listed in 802.11k/v/r Amendments
Could you elaborate the steps please?
Do you have app for Android?
There is youtube videos on xiaomi routers, its kind of trials and errors because the app is chinese.
Don't know any Android app that does that
I can tell you the roaming is now working very well for me with Dawn . Even without 802.11 r/k/v .