Can you check that your system clock is not out of sync by a few seconds? Cookie expiration uses absolute time, and these cookies have very short expirations.

I'm on macOS with the system set to automatically keep the clock in sync using Apple's NTP time.apple.com servers. I don't know how much that can be assumed to be out of sync for generally, but this is the system default and I've not changed it since it was setup four years ago.

I suspect that either the server-side clocks are somehow behind more often than usual, or (much more likely) that code execution has been re-ordered or slowed down in MediaWiki such that the cookie is drafted in PHP very early on and thus already closer to expiry by the time it arrives. That, or something regressed in its logic making it off by a few seconds more generally.

I don't know what else could cause this to be so common recently, but I suppose one way out of it is to solve T274724: Add support for "Max-Age" to WebResponse::setCookie (more reliable than "Expires").

If I recall correctly, the cookie is limited to a given title/revision (so longer lifetime would not make it appear in unrelated places), and it is also unset upon first view (so longer lifetime should not make it appear multiple times). Perhaps the expiry could be increased to encompass most cases of slow server responses, network latency, and clockskews, possibly even going up to an hour or longer (see T206804 and T196248 for clockskew data).