Maybe a bit confusing but "Resize OSD and subtitle font automatically" has the following helptext (shown if you put mouse pointer at the option)
"If checked Subtitle and OSD fonts will be automatically resized accordingly with playback resolution"
This is a different feature from "Auto resize font if line is to long".
So what you're showing with your pictures above is that this second option works as it should
(On the other hand you do not show that the playback-resolution dependent resizing doesn't work).
(P.S.:
I wouldn't generally consider a subtitle size of 48 as 'normal')