In T344189#9093734, @TK-999 wrote:

Possibly. I think it's worth disabling the JIT (ie making sure that opcache.jit_buffer_size is unset or 0, which is the default) though just to rule it out as a cause—it's been known to cause oddball issues when it is enabled that simply do not occur when it is off, and its maintenance status in general is dubious.

In T344189#9092851, @TK-999 wrote:

Might be worth an upstream report if a minimal reproducer can be isolated.

Are you using the JIT, just in case?

Apologies, I thought I had all other skins disabled, it seems I still had a bunch enabled. I found the issue with two skins (at least two I have installed): Metrolook, and Tweeki.

In T336643#8849363, @Ammarpad wrote:

Have some skins loaded

Please give precise skin name.

In T322959#8390906, @ashley wrote:

@Ladsgroup So while investigating this ticket I stumbled upon T307616 and your dd9b44b167141, which removed the useIndexClause method but it looks like it was accidental as there was no mention of it in the commit message, and the method was neither deprecated nor marked as private. Can you confirm whether this was truly the case, and if so, could the method be added back to the relevant core class in order to fix this issue? Thanks!

Is there any more progress on this? This seems like a simple request, that has not been in almost six years, and no activity at all in 2 years.

Some of this already mentioned by @ashley above, but just reiterating with some other ideas and how this can potentially be done.

Thank you for this task.

In T311053#8117975, @Nikerabbit wrote:

When the issue happens, I see x-cache: cp21 HIT (4) or similar in the response. I think your Varnish is not configured correctly to split the cache.

I'm closing this task since there isn't evidence that this is a bug. No such issues has been reported from e.g. those Wikimedia sites where this option is enabled.

This same issues has been reported to us multiple times.