2019 Tundra cold start issue

Hey everyone,

I wanted to ask and see if anyone has seen this issue before and what the problem could be. I have a 2019 Tundra with 242,000 miles that just had a new heat exchanger, thermostat and serpantine belt installed due to the common valley plate leak. When I start it after it has been sitting for several hours and the engine is cooled off I notice the alternator voltage gauge on the dash is reading 12.5 volts. It does this for approx 5-10 seconds until what I believe is the SAIS kicking on (the hair dryer/blower sound for 5 seconds). When that happens the voltage gauge on the dash immediatley returns to the normal 14 volts. Took it back to my mechanic and they cant find the problem. Alternator and battery were checked and appear to be good. Would a disconected plug some where on the engine cause this or a blown fuse from a short? The battery was disconected when the work was done on my truck.