You may well be correct that this is an issue with the theme. Can you switch to one of the default (Twenty*) themes to check?
If you can install plugins, install “Health Check”: https://ww.wp.xz.cn/plugins/health-check/ On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.
I see the same (bad) behavior when I activate the 2020 theme, just for new post with the problematic gallery. Older posts look fine. And I created a new post with a 3 column gallery with 2020 active, and it has the same behavior: all image thumbnails appear in a single column.
Thank you: trying the Health Check plugin was the perfect advice! My hosting site optimizer plugin appears to the culprit! (SiteGround’s SGoptimizer plugin).
I am having the same issue with multiple websites/themes, with limited plugins and no other gallery plugin
https://woman2woman.co.za/gallery/
Eventually I was able to clear the SGoptimizer plugin cache, reset the plugin settings, etc. and the problem now seems to be fixed. Definitely not a browser issue, but the plugin bad behavior somehow, caching or settings… unfortunately I cannot be more specific. Presumably other plugins could do the same… 🙁
