I think maybe I'm onto the culprit here... I just played around, and happened to type "a" and it completed to "afternoon" (but nothing else). "b" won't complete to anything; "c" completes (only) to "continuous"; "d" to "day", "dusk" and "dawn"; "e" to "evening"; "f", "g" and "h" to nothing; "i" to "intercut"; "j", "k" to nothing; "l" to "later". Already I'm having a strong suspicion -- but "m" really nailed it: It completes to "magic hour", "morning" and "moments later". 10 points for guessing which bundles I just installed... ;-)
So -- I'm guessing that the "time auto-complete" feature of the Screenwriting bundle has somehow replaced my ordinary completion (as I get the exact same behavior when using option-escape, which is the actual keyboard shortcut for that).
I'm guessing that this is a bug in the Screenwriting bundle (or, perhaps, somewhere else) ... Should I simply uninstall the bundle? Would that even help? Where would the completion function have been (erroneously) overridden (i.e., where can I change it back)?