@rho-bot: I doubt it’s the boot loader in this case as with an EXT4-formatted external hard drive the issue does explicitly not occur. That’s been practically verified right now. It’s completely BTRFS-related. Beyond, we’re not talking about a file transfer here - just about unmounting a BTRFS file system.
@rho-bot: It’s even worse. When writing the referenced bootloader image to the LePotato CoreELEC 19.3 block device “/dev/mmcblk1” it does not boot anymore and results in a black screen:
dd if=/storage/bootloader.img of=/dev/mmcblk1 conv=fsync bs=1 count=112
dd if=/storage/bootloader.img of=/dev/mmcblk1 conv=fsync bs=512 skip=1 seek=1
Finally, giving the bootloader a try was a bad idea as it seems not to be compatible with CoreELEC 19.3 on LePotato! Now, I have to re-install CoreELEC 19.3 again 
So, the current work-around is to switch the used file system on the attached hard drive from BTRFS to EXT4 - and the issue has gone 
Even better news added here on 05/30/2022: starting with CoreELEC 19.4 the CoreELEC team bumped Linux kernel to 4.9.269 that sustainably fixes the BTRFS segmentation fault issue!
Great work CoreELEC team!!!