and evidently, the Pro Mini bootloader doesn't handle the wdt correctly. See https://forum.arduino.cc/t/atmega328-locks-up-on-watchdog-reset/921269 . Some people say you can fix this by putting an Arduino Uno bootloader on the Pro Mini, and thereafter treating the board as ...
The whole source is 1.4G, of which you’ll only need 16M, so you may just want to use the sources installed by the IDE. On Ubuntu, see /usr/share/arduino. I recommend making a copy as you’ll need to make some changes. Change to the ./hardware/arduino/bootloaders/optiboot directory...