…mmm the solution there was to drag a window of files over to another Mac with a linked screen extension, and then the thumbnails might be visible. Crazy.
Using SU2025 and resizing the thumbnails, of files saved in SU2025, didn’t work in the end, only for files saved in SU2024. So I’ve gone back to SU2024, seems to work ok with the thumbnails, even when using the open dialog box in SU. Something about SU2025 and OSX is just not getting along. I’ll stay with SU2024 until I hear of a proper fix with SU2025!
SU2024 is the current fix.