Hello, in 3.4.1 and latest (3.5 3505) I get the same issue:
if I have a backup layer and do “use as master” and then undo, then redo it (with shortcuts Cmd+Z, Shift+Cmd+Z) all the backup layers disappear, the master layer is no longer bolded. If I save (or not save) and quit and re-start the app, the layers are still here but the backup layer is now called the name of the master.
Possibly related (but this is an old bug): Selecting an intermediate layer, then pressing the + button to create a copy, it is not named with time/date but it’s called “(empty)”. Also, it gets inserted above the layer, rather than below.