most rentals don't use a physical transponder anymore, the cars are 'fleet registered' under toll by plate where the system captures the tag number and bills accordingly. If you use a transponder of your own, it overrides 'toll by plate'.
all explained here:
https://www.sunpass.com/rentalcar
In simple terms an electronic toll bridge or lane will first look for a transponder to process. If no transponder is found the system "captures" the tag. First they look at SUNPASS to see if the tag is attached to a transponder, then they look to see if it is registered to a fleet account, and lastly they process as unregistered.
This is why it is important to register the tag in SUNPASS, and not simply rely on the transponder "working". Similarly, when you return the rental, remove the tag from your account promptly.