User builds on some Chinese phones are pretty sloppy. I needed to access an old Oppo phone the other day, where I couldn't remember the PIN. Luckily ADB was enabled, which suggests that their production software might have been a userdebug build. I couldn't enable root via ADB, since it was at least a production/user build, but the su binary was already on the phone, so I just su'ed and got a root prompt. From there I could pull the sqlite settings database, reset the PIN and push it again. After a reboot, the phone booted without PIN.

Unfortunately there was an Oppo homebrewn secondary PIN on some of their built in apps, which hadn't been reset, but it turned out I could enter the PIN as many times as I wanted, so I made a small script to brute force it via ADB (input text). Took half an hour to disable the secondary PIN with my script.