Yes, that is exactly what is happening for me.
I’ve implemented a different work-around for now, by adding a rewrite in the nginx config (at server level) as follows:
rewrite ^(.*)$ $1?safe_mode=no_themes break;
So visitors will use safe mode by default now.
To allow further exploration of this issue, I have also duplicated the forum server without the rewrite at https://raw.forum.embeetle.com. If you visit that URL, you will still see the original issue.
I am not using any custom scripts. I am using some plugins, will experiment if disabling any of them has any effect.