Caught it while scrolling all! Not really scrolling fast but there's another common variable... It was for a video clip.
See the "before tapping" in attached.
It changed to the correct thumbnail once tapped and backed from. See mildly edited next:
Not sure it'll help but that post seems to be here:
(Obviously not safe for work)
https://lemmynsfw.com/post/28850749
Last I confirmed version:
Edit: scrolling back just a few items yielded the original image too I think:
That's for https://lemmy.bestiver.se/post/535254
Edit 2: holy cow... Ummm...
So I had scrolled back up to find the image and after editing here, continued scrolling down (4 posts down by the way!) and this is what I see on that nsfw post preview now:
Worth noting that that image now in that place is from not far back also. 5 posts previous (1 before the first source image).
Also, once hitting save to this edit, the preview was back to the correct NSFW thumbnail.
I hope this helps track down the glitch!
Found another issue maybe... Attempting to post the comment after it has been saved as draft and there's no way to post, only save? I had to come back to post directly after copy/paste.
The issue seems to manifest most frequently on posts from lemmynsfw for me when I'm scrolling under all.