there are few suggetions for people that has this bug:
For example you have already made model with hires modifier and you want to use Apply Base on it. Like this one

If you just press that button it will ... destroy your sculpt -

But! If you first sculpt something on model ... something, whatever it is

And only then press Apply Base - everything will be ok.

This is after i pressed Apply Base. as you see all sculpt data is on it's place. But topology of mesh is changed. Probably when blender activates sculpt mode it is not load all the sculpt data or... something like this.