Hi @GeoffreyT2000, thanks for taking the time to report this. Please always use the bug report form for creating bug reports instead (it is linked from the preamble), and structure your bug reports.

Up until recently, oldid views were not cached on the server (only in one's browser afaik, if that).

A few months ago (as part of T244058), a secondary ParserCache was introduced for old revisions. This is configured via wgOldRevisionParserCacheExpireTime which is set to 1 hour (compared to 30 days for current revisions of articles). The current-revision 30-day cache is purged after an edit, after a red link is created, after a used template changes etc. The old-revision cache does not get purged and this is by choice since it lasts only one hour which seems short enough. If anything, people might expect it to last longer and thus cause to change. (There are numerous requests for making old revisions stay exactly as they were in rendered form, since now when they expire they can start to fall apart of templates change etc.).

If an old revision cache lasts for more than 1 hour, then that would be a bug.