Same thing happens on the Poco F3 and didn't occur on the global ROM while waiting to unlock, except mobile data drops entirely so you can't send sms or use data (every app will say you're not connected to a network and calls won't even complete). The fix for me was using root+network signal guru to uncheck band B7 in preferred networks on the two F3s I own. Another guy had the same issue and had to uncheck band B66 (but that was already unchecked on both of mine). I suspect it has something to do with global vs china roms but now that mine is behaving and I'm working on the road, I don't want to switch to a pure china rom from the .EU rom to test. I just know that like you, I didn't have constant data drops on the global ROM. At least the network signal guru fix worked for me.
It's probably something wrong with the china base and not something the .eu devs did, but it's hard to say.