Digispark ATtiny 85 erases flash after power loss
I got the same issue, on two boards. Will probably try to burn the bootloader using usbasp, maybe they burned the wrong one back in china.

EDIT:

As expected flashing the micronucleus bootloader using USBasp solved the problem on both of the boards. I've used burn-o-mat as the arduino was not seeing my programmer.

Just download the firmware from github (I've used this: https://github.com/f4goh/micronucleus/archive/master.zip )

and connect like so (here the guy is using arduino uno, doesn't matter): https://hamprojects.wordpress.com/2018/12/27/digispark-microcontrollers-implementation/