Where you see that other style, I assume that you are not connected/logged in?
That is the new theme of the forum, which has the advantage of being mobile-friendly, but I don't know how to apply it to existing accounts I'll ask to the forum owner.

I have access to this site from work and found this layout format like this

Discussion about images submission split to a new thread: https://imcdb.opencommunity.be/forum_topic-9967-93534-Submitting_images.html#p93534

Minor update, following a request received, now in the page that appears when one uploads a picture for comments you can simply press the Enter key to copy that URL to clipboard (in addition to Ctrl+C or the "Copy" button at is side).

I suspect issues with some ads (I really should change the ad company), even if not optimized on mobile it should work fine, as it is rather basic pages.
This site is a relic from the 2000-2010s internet It was a little tweaked to make it more usable on mobile, but it is not a mobile-first site like most of the sites nowadays.

Just wanted to chime in and say that the mobile site is very unoptimized on Safari on iOS. Sometimes I can't click on links or even the text box if I need to edit a comment.

Small update for the popup of comment images: if a timetag can be guessed from the picture name, it is displayed. There may be cases where it is wrong in some cases, or not detected where it should, I can try to tweak the code if needed.

You can check in the thumbnails I post, e.g. in https://www.imcdb.org/yourcomments.php?formovie=33550045

I test mostly with Firefox and I didn't had that problem. Maybe it is due to some ads? You could try blocking them just to check if it is caused by that (as they depend on the country, maybe you get some that cause issues)

It happens with both but it's far more noticeable in vertical mode.

I use Firefox.

Do you mean that it happens when you are in vertical mode or in horizontal mode?
Which web browser do you use?