Keep noticing when applying brake whilst travelling that the brake cylinder either empties to zero or rises slightly then decreases to zero, when the train stops brake cylinder fills, then sometimes empties despite full service application noticed on several services and locos One loco I noticed in particular was the DB BR 182 HBL
Pretty sure what you're experiencing is normal behaviour for blended brakes - the air brakes come on first as they respond quickly, then taper off as the dynamic/electric brakes take over. As your speed decreases the electric brakes become less effective, so the air brakes come back on for the final stage to bring the train to a complete stop.
