This is a problem because a plain index.php causes the next search performed to revert to the default search namespace, and all subsequent searches will default unless alternate namespaces are selected in the checkboxes. Normally, the selected namespace would be remembered for consecutive searches, but this means the namespaces are only remembered for one search.

Here's four examples of the bug being triggered (some use the article namespace and some use all namespaces):

• hastemplate:Notsure intitle:"GA1": remove intitle:"GA1" from the search.
• hastemplate:Notsure: add intitle:"Articles for" to the search.
• intitle:"GA1": add hastemplate:"monkey" to the search.
• hastemplate:Notsure add either intitle:"GA1" or intitle:GA1

The fourth example is particularly weird, because when I first noticed and started documenting this bug yesterday, those searches worked fine. I don't think the codebase on the English Wikipedia has changed in the last day, so it's possible there are different contexts that affect when the bug triggers, though I can't imagine what those could be.

More specifically, the difference I've noticed between today and yesterday is the following sequence now sets the URL to a plain index.php, when it didn't previously:

1. hastemplate:Notsure
2. [switch namespace to all]
3. hastemplate:Notsure intitle:"GA1" or hastemplate:Notsure intitle:GA1

But this sequence still doesn't:

1. hastemplate:Notsure
2. hastemplate:Notsure intitle:"GA1" or hastemplate:Notsure intitle:GA1

And here's another example that didn't previously trigger the bug, but now does:

• hastemplate:Notsure intitle:GA1: remove intitle:GA1 from the search

I say this is a recent regression because I often refine my searches when I'm searching, and I didn't have this problem when I did some searches on May 6. So it was introduced to the English Wikipedia somewhere between May 6 and May 17 (yesterday as of this writing). It seems likely this regressed in this recent MediaWiki update, though it probably was caused by some update to CirrusSearch (disclaimer: I don't know the codebase very well, so I may be mistaken).

This bug makes it extremely difficult to revise searches, and it was introduced recently, so I hope this can be prioritized appropriately.