boot loop – Phone doesn’t work with custom ROMs after flashing Flashboot Firmware to recover from a bootloop soft brick. What could be wrong?

I have a Xiaomi Redmi 5, which I recently unlocked, rooted (SuperSu), added TWRP recovery, and then flashed a Custom mod (lineageOS). Everything worked out well, and the device was working flawlessly… I then realized I needed to repeat the process because I didn’t know I had to flash Open-GApps right after the mod, for it to work properly. This time too everything was working well. I clearly had a lot of time on my hands, because I then thought I didn’t want Open-GApps after all, and decided to flash a third time reverting back to the vanilla LineageOS setup. This time I goofed-up majorly and wiped the whole system and tried to reboot into a system with no OS and ended up in a bootloop…

After doing some research I then flashed the OEM Flashboot Firmware to successfully unbrick the device, and regain full functionality. Only, now when I flash a custom mod, using the same files and same process from before, the installation process goes without any hiccups, and I can also boot into the Custom mod; but the phone just switches off after a minute. This happens irrespective the Custom Mod I am trying to install (to find out if it was some incompatibility with my LineageOS, I tried ArrowOS and Paranoid Android, too).

I don’t know where to begin looking for hints. Tried to use to TWRP terminal to look at the logs for hints, but nothing suspicious there.