If I want to interpolate between background and foreground, it seems to work fine in preview, but when I click OK, the glyph goes back to how it was. This has happened to me several times, at seemingly random moments. I can send a short screencast to show the problem if you want.
Cheers,
Jasper
What version of Glyphs do you have?
Version 2.4.1 (965)
But I had the same problem in older versions.
You cannot reproduce it reliably? 
Nope, sorry
I’ve been having this problem quite a lot lately.
The background interpolation is previewed but clicking OK (or hitting the Return key) returns the glyph to its initial state. Glyphs is registering as having completed the action though, as hitting Cmd-z afterwards gives you a report of ‘Undo: Transform’ at the bottom of the window – although nothing is actually changed.
What does occasionally work for me is to apply the filter with no nodes/handles selected. I’m in the habit of hitting Cmd-a to select all nodes/handles before applying filters, but doing this before interpolating to background seems to make a difference sometimes.
Yes, that also works for me sometimes, though (I think) not always.
Can you send me a glyph that behaves like this?
Also been having this problem, usually when attempting to transform only selected nodes, rather than the whole glyph.