This was reported for itwiki filter 529. If you try to set a very short block duration (e.g. 15 minutes), no block will be issued.
I've already investigated, this happen because we pass -1 to SpecialBlock::parseExpiryInput, which in turns returns (IIUC now() + 1 hour), hence the duration provided by the "current" filter is deemed shorter and discarded.
Easy fix, coming soon.