preface: it seems pictures made with the Pixel 7 Pro aren't shown correctly as an image post, but only in the post body or comment, when inserted.

as it seems that it's not the file size, I've now cleaned the meta data from the image with 'exiftool -all=remove'

edit: surprisingly that didn't work. here the link to the uploaded image:
https://sopuli.xyz/pictrs/image/19621e35-1354-4e68-816e-f5b44a663495.jpeg