Hello @Dan_Merry!
Unfortunately Victoria is no longer on the team 
This has been an issues faced by other Retool users, one work around that had been suggested was using RAW mode for the query.
There is another forum post on this issue that might have some unique work arounds that could help.
From my understanding Retool sends timestamps to Firestore, but on their end the timestamp is being converted to a string, which unfortunately we are not able to control.
There are options in the forum post I linked above for converting the strong stored in Firestore back to a moment object once it is fetched by a Retool query.