That is indeed an annoying effect, too.
Have you tried various Safari routines..?
(Any chance you may have turned on 'Safari Developers' menu in Browser?)
Clear your browsing history in Safari on Mac - Apple Support
These could have a desired effect ~ to return to Safari to defaults.
• Safari User Guide for Mac - [Table of Contents] - Apple Support
//support.apple.com/guide/safari/toc
There also are 'reading list' features; never used those custom settings.
A larger volume of Safari support articles are linked within User Guide.
Keyboard shortcuts and gestures in Safari on Mac - Apple Support
(exit full screen view in zoom?)
• How to zoom in or out on Mac - Apple Support
//support.apple.com/en-us/HT210978
• Zoom in on webpages in Safari on Mac - Apple Support
• How to Set a Default Zoom Level in Safari for Mac - How-To Geek
https://www.howtogeek.com/658782/how-to-set-a-default-zoom-level-in-safari-for-mac/
By accident, had same effect in iPhone iOS, but that never stuck there.
Could be useful to archive or print several long messages in iOS.
The Safari 'Reading List' article has some interesting aspects; those may
not apply to this instance however, but to re-set the browser could help.
Set up iCloud features on Mac - Apple Support
Maybe something was set in iCloud preferences, those effect across
several devices; IF you see that happening on other Safari versions.
..Sorry to not have any exact cure that may pin-point a cause..