I have a device running Android 10. The bootloader is unlocked. I was able to flash TWRP but there’s something strange happening. I guess it’s because the manufacturer has enabled boot time USB stick firmware updates. So, they have put a countdown timer during the boot time. Normally, the device starts, periodically it gets into “update mode” where it waits for 10 seconds for a USB stick with an updated firmware and if not found it boots into the Android OS normally.