Never been an issue? Just recently there was a LTT review of an Aliexpress laptop [1] where they found the Windows install had been tampered with (mostly because the Windows was pirated). PC or phone or anything, you buy stuff from shady sources it will not be trustworthy. If Xiaomi is/was locking down their phones to try and make purchasing from places like Aliexpress safer, I think that is a losing battle.
I bought a laptop from Dell and found the Windows install had been tampered with; there were all sorts of adware and trial software installed. Luckily, since the boot loader wasn't locked, I could install my own, clean OS.
I don’t think that’s the point, when you get a device from Aliexpress that’s been tampered by the retailer (as opposed to the OEM), the bootloader will already be unlocked and you could install a clean OS. That’s okay if you are the one buying the device and somehow trust that the retailer hasn’t messed with the hardware as well, but it doesn’t help the manufacturer that wants to stop retailers from messing with the OS in the first place.
[1] https://youtube.com/watch?v=Um0l6ZSxfTE