You will be fine, we all are. Your bootloader will show open but will not be detected. That's the nice part of the official custom ROM made by the EU ROM Devs. Just make sure you do not use miflash. Use twrp or fastboot script in the ROM download and if it is the first time, format data as there will be some partition differences and it may not boot without format.
 
I'm on weekly 13ultra and mine says not certified but works fine with Play Store. I can buy apps with no issues so I doubt if it is this.
Hi. Do you have some usable info/update here? I'm asking because i have the same Problem. My phone is suggenly not play protect certified. Previously it was no problem to play contactless with Google Wallet using the built in NFC chip.
 
Hi, I bought the mi13U Chinese rom 16/512 version & did not unlock my bootloader or flash my rom to global or EU. I install google services, google play & uninstalled all the Chinese apps via PC. I could use NFC & google wallet to make payment. The drawback of this is language is limited to English...but everything works except android auto. Few days back i got updated to HyperOS. Xiaomi's version of android auto is known as "CarWith" and the list of Chinese cars get longer each time. Why would I want that is because with the amount of Chinese EVs now flooding the market, I would likely buy one & the CarWith app in HyperOS might come in handy...