Dean spent most of “Last Call” dealing with a standalone case that involved a duet with guest star/surprise villain Christian Kane, while Castiel dealt with accidentally hurting Sam worse than ever when trying to probe the wound from when he shot God, which resulted in Sergei trying to coerce Cas into assisting him.
Supernatural took a couple weeks off in Season 15, only to return with an episode that proved Dean and Castiel really need to make up sooner rather than later.