Some little ones can go through stages of defiance. It's almost as if they are doing it in their pants because they can control it IYKWIM.
I've had these issues with both of my boys.
The way I've gotten through it is rewards (or bribes ).
A choc bit or you could try a star chart. If you're worried about the chocolate I can tell you that after a while they run out or I "forget" to buy some more. Hopefully by then they are in a toileting habit again.