Possible bug: Categories change randomly for some games when non-Steam games are included

[r6d2]

If I have categories named "Collection 1" and "Collection 2", both including Steam and non-Steam games. Then:

1. I close Steam.
2. I do any change within Depressurised (even if it does not involve those 2 collections). Close the program (exporting to Steam).
3. When I reopen Stem, the games are scrambled on the two collections: some of those from collection 1 are in collection 2 and vice versa.

Is this a bug or am I doing something wrong?