Hey there! That’s a super weird issue – do you happen to have a caching plugin installed? I’d try deactivating that and seeing if that makes a difference.
Otherwise, I’d recommend switching your theme to a default one, such as Storefront or TwentyNineteen. Does the same issue happen when such a theme is active?
I do have a caching plugin (which is pretty useful, the site is incredibly sluggish without it) but I cleared its cache to no avail many times during my trials.
I also cleared Autoptimize’s cache.
The theme is Astra, isn’t it pretty common?
(I fear I’ll break everything if I switch theme :D)
Astra is common, yes – though unfortunately we also encounter quite a few issues with it 🙁 comes with the turf, I suppose. In any case, it’s understandable to be apprehensive of breaking your site. Testing like this is best done on a staging site. That’s a copy of your live site, so you can test things out without making changes to your actual site. Some hosting companies offer a staging site service within their hosting plans. If yours does not, there is a great free plugin that will allow you to build a complete staging site with a few button clicks: https://wordpress.org/plugins/wp-staging.