Okay I think this is a bug then.
By cell-size, I meant the size of one frame within a sprite sheet. When importing a sprite sheet into a collection, the Config panel asks you for the size of the frame.
What appears to be happening is that the sprite collections aren't retaining that information when displayed within the Sprite Animation editor. If I switch back to the Sprite Collections editor, everything looks fine. If I click commit within the Collections editor (even though nothing has changed), it "fixes" the issue in the Animation editor, but that causes other animations within the Animation editor, specifically those with frames from different Collections, to display the problem.
I've tried vising each Collection in turn, but as soon as one is "fixed", another one that was previously working is corrupted. I only have three collections.
Thank you for any further assistance.