[Nail Grease]

All Honda Pilots have a timing belt and a serpentine belt (drive belt) that turn the alternator and AC compressor. These both are unrelated with down shifting. If your experiencing a hard down shift, I'd take it to the dealership to check it out.
What is the current milage?
Is it time for the transmission fluid to be drained and filled? Servicing the transmission on time with fresh fluid is very important.

 

[Jack Vogt]

A 2021 Pilot should not need any transmission fluid changes yet. It could be the computer is still learning your driving habits. But it could be a problem somewhere else.

Either way, the car is under warranty so the best thing to do would be to take it to the dealer and have them figure it out.

 

[oseto317]

What speed are you at when you left off the gas pedal relative to what cruise speed you have set?