We were facing this issue as well, but it is now working again after upgrading prod to 17.4. However, before this the new posts were prefaced with [NEW] and this went missing at the time we started experiencing the bug. Lithium Support is still looking into this.
Thanks for confirming the fix, @lilim
Regarding the "[NEW]" prefix: Where was that shown? The Forum Topic Page and there on the subject line for each reply or are you referring to new, unread topics on Forum Board Page?