Rheem - Model #: RRGG-05N31JKR - Furnace Problem

Hello, Following behavior occurs when the furnace turns on: The blower starts immediately when in HEAT position. (No time to make the fire process first before blower starts). I am not sure if it depends on the unit and is it normal that the blower starts immediately in Heat position? Warm air is coming out for approx. 5 minutes, then I can hear that the compressor??? (also the big fan on the top of the unit) starts running (as when it were in A/C mode), so suddenly the warm air turns to room temperature air with no heat although the heater is still running and there is fire in the unit. You can feel the heat through the inducer motor coming out on top of the unit . I also recently replaced the blower relay and the FCC. See attached pics.