If for some reason the HEIC images are what got transferred to the Mac, instead of them being converted to jpg, then that’s probably the issue. The Automatic only works when transferring from the iPad/iPhone. That, of course, leaves the question of why the format was not changed when transferring to the Mac.
I always upload to the forum from my iPad or iPhone. Since I don’t have a Mac handy, I can’t test what’s happening there.
As for settings, mine are the same as those you show, except I have Summarize Photos turned off. That should not make a difference. Having iCloud Photo Library turned off should not make a difference when posting from the iPad or iPhone. It might make a difference if you were using it to sync to the Mac’s Photos app, but you are not, so still don’t now why your photos are transferring to the the Mac as HEIC.
If you are using the Mac’s photos app, then you might want to check it’s settings for a similar option. If you are directly saving to and uploading from a folder, then there is probably no control on the Mac to automatically change formats. Though a through perusal of Safari’s settings might show something.