When you exchange tp with flash (in cd, like 10 s cd left to re-use flash), after u use the tp you must wait all tp coodown. It even stays as if you have choosen tp as the primary summoner for all the cd. After all that time, it turns back again into flash (with description and everything).
Testing steps (as I found the bug)
1. enter game as kayn
2. jungle until min 5
3. use flash
4. at 20-10 sec cd left to regain flash switch it to tp
5. use tp on an ally minion