This thread is quite old (around 2 years old). But nevertheless, I am sharing my experience so that it may help someone facing similar problem. I have a Moto G 1st Gen Falcon which was running Lineage OS 14.1 based on Android 7.1.2. I was planning to update it to Android 10 based Unofficial Lineage OS. I downloaded latest TWRP zip to update existing TWRP. I rebooted into TWRP and flashed the zip. It installed perfectly. Then to check it attempted to reboot into recovery. But it instead went to fastboot mode with the error "Invalid boot image size! failed to validate recovery image Boot failed". I tried botting into recovery but was not working. I tried flashing through adb a different file but it was not working. Then what worked was booting into recovery using the command Fastboot boot recovery recovery.img. After that I was able to flash Android 10 and now everything is working perfectly.
