Recent work on this feature introduced an annoying regression.

From emacs -Q:

  1. M-: (erc-tls :server "testnet.inspircd.org") RET
  2. /JOIN #test and say something
  3. M-: (setq erc-timestamp-last-inserted-left nil) RET to reset the
     date stamp's deduping snapshot
  4. Say something else
  5. Notice that point has been dislodged from the prompt and that a new
     date stamp has not been inserted

The second of the attached patches should fix it.