In the latest firmware the heaters will temporarily shut off during G28 Z and G29 to reduce electrical interference with the sensor.
If your heater temps are dropping too much you can add a “wait” command after G29 to let everything heat back up. To do this add the below code AFTER G29. The heaters shut off to eliminate electrical interference with the probes.
G4 tells the printer to wait S10 is seconds. So G4 S10 will wait 10 seconds, thus letting it heat back up. If you need more heat up time just change to G4 SXX – XX being the seconds you want it to wait after G29.