"I wonder if the behaviour of the former syntax is new, broken, or something I just imagined was different."

Unfortunately, I suspect it may be the latter.

I rarely, if ever, use those shortcuts; however, checking in "What shortcuts can I use for linking to other information?", I see (my emphasis added):

The Perl documentation:

• Search the Perl docs: [perldoc://search terms]
• Link directly to a the doc of a function or variable: [doc://name] ...

There was a change to [doc://name] back in February: "NEW [doc://perldoc#deeplink] works now with all perldocs". I don't recall any recent changes to [perldoc://search terms].

"Maybe a topic for PM Discussion..."

Or maybe not. :-)

Unless you wish to request a typo fix: s/to a the doc/to the doc/