Didn’t see any screenshots.
This sounds related (not identical) to Project list sort strips away all my due dates. No option to undo either In that case, Asana indicated that the behavior of due dates being erased was intended. 
The T/U/L field, in a sense, “fights” with the due date field in the context of sorting, and Asana had/has to choose just 1 winner. (Wish they would have given a warning and/or a chance to undo.)