A bit of guesswork here, but: back in ancient times if you flipped or rotated an image your computer would actually unpack the compressed image, transform it, then repack it and every time you did that it added to the JPEG Crunchiness, so more modern transform software instead just tags that the image is rotated and the viewer software renders it that way. My guess is that the software that makes the thumbnails doesn't know how to read that tag. Thanks for coming to my TED Talk.

Same thing happens to me. Wild.

That's weird...and so is the flipping

Yep

Yes, it is mirrored when I opened it as well

Same happens in Eternity

Yes. It happens for me both in Sync and in Eternity.

Same for me. I'm on Android 13

Yup.

But I blame the giraffe.

Can't trust them. ;)

Dancing giraffe

Not I. Both face right

Yes.

I've also had images rotate compared to their thumbnail.

Device information

Sync version: v23.09.13-18:19    
Sync flavor: googlePlay    

Ultra user: true    
View type: Smaller cards    
Push enabled: false    

Device: ASUS_AI2302    
Model: asus ASUS_AI2302    
Android: 13

Thumbnail and link both face right. Using Sync.

Neat.

They flip for me

Device information

Sync version: v23.09.13-18:19    
Sync flavor: googlePlay    

Ultra user: true    
View type: Slides    
Push enabled: false    

Device: d2q    
Model: samsung SM-N975U    
Android: 12

Happens in Jerboa

Wild