Cell styles are not saved / are reset

I have a spreadsheet in Calc with multiple sheets where I set up various formatting conditions with custom/modified cell styles. My problem is, that these styles somehow regularly get reset so everything is messed up in the spreadsheets. I have these cell styles saved in a template which is set as default.

Example: I created a style which I called “white on white” (just white font and no backround) to hide some values e.g. “#value!”. So I opened a new spreadsheet from the template mentioned above, copied/entered the values from the broken sheet, redid conditional formatting and saved.
Next day I open the file all cell styles are gone/reset - only the ones that are default when you first install LibreOffice are there plus three styles called “ConditionalStyle 1” to 3 which are basically the styles I created/edited but not entirely; the style “white on white”, which is now called “ConditionalStyle 1” has a white font plus white backround now, which results in a big white block if neighbouring cells are affected, because the cell borders are gone.
There is apparently also no option to apply the styles of the custom template or reset them. This is extremely annoying because setting up the spreadsheet was a lot of work and everytime I open it I can redo it partially.

I use Ubuntu 17.10, LO and xlsx as format. The sheet is saved on a usb-stick and synced by MEGA, but I guess that shouldn’t be the cause of the problem

My attempts so far: Purge LO completely and reinstall it, create a new sheet from the custom template and copy everything from the broken one then redo formating like mentioned above, modifying the styles to match the template also results in the same problem

Thanks for any kind of help.