Sorry it's taken a few more days to reply @cmooney , this cold kicked me harder than I thought. Feeling better now though.

I'm the user from Northern Ireland who sent the email cmooney copied above. I'm currently struck down with some sort of cold or sinus infection from my nephew, so I'm not able to do a full test right now as plugging in my laptop is more energy than I have to spare. I did a quick test trying to load enwiki on my phone, over the IPv4 only connection and it was loading consistently fine. If I feel better tomorrow my time zone I'll rerun the tests from my email above and post the results here for comparison.

I'm more than happy to @ovasileva !

Would it be in scope of this task to also make it configurable (even if only to logged in editors) as to which of the two sidebars the floating tools menu docks itself into? I'm sure I'm not the only editor who would prefer to have it on the left hand side, either before or after the table of contents.

Did some more testing, seems to be JavaScript related, as disabling all scripts via NoScript results in the expected behaviour. Unfortunately I can't dig any deeper to find a culprit script, as I've not done any webdev in a hot minute and my skills are a bit rusty. However as it seems to be occurring in safemode, it suggests that its one of the scripts that come from MediaWiki core and not a userscript.

From my own testing using Firefox 103.0.2, Chrome 104.0.5112.102, and Edge 104.0.1293.54 on Windows 10, it appears as though the page is initially loading and locating correctly to the anchor link, and then once page load completes the browser jumps back to the top of the page.