Well maybe that's because it's the only rational explanation after waiting 10+ years for a seemingly basic feature.
And what's venting air seems to be voting for this feature, as user votes seeem to be completely ignored by the devs 😐
We're in 2022, still no way to do this?
As people have said, if you cannot allow the user to pick the thumbnail for whatever reason, then at least change the heuristic used to pick the thumbnail and pick the first thumbnail in the note. And if the design is so bad that you cannot find the first thumbnail of the note from wherever that code runs because they're not accessible in sorted order in the database or whatever, then do not display thumbnails in notes ! or allow to disable that broken feature. I'd much rather have no thumbnail than a random image which is both distracting, misleading, and even though inside the note, might be irrelevant to the note preview in its globality (when you have 20 images in a note, clearly, this happens)
How to frustrate users & send tables flippin 101...