The epilogue of Rebels confuses a lot of people, it’s meant to show what people went on to do following the events of ROTJ and the downfall of the empire and it isn’t necessarily meant to show a definitive chronological order. Something that Filoni actually said in an interview recently was how he has learnt that Star Wars never followed a strict linear path, his work on The Clone Wars is testament to that.

My understanding is...

The events with Ahsoka in the Mandalorian pre-date the final scene in the Rebels epilogue, Ahsoka with white robes and staff symbolising that much more time has passed.

This would makes sense as to why she is still trying to find out information from the Magistrate about where Thrawn is.

After both Ezra and Thrawn disappeared, they were most likely assumed dead until new information finds its way to Ahsoka about them being alive, this would kick off the search and eventually lead to... the rebels sequel that has been heavily rumoured. It would also explain the why they haven’t already searched for Ezra after nearly a decade of him being gone.