BE36 pilot flying an assigned heading from Center reported an NMAC with a descending VFR aircraft.

Narrative

The below narrative is related to a near mid-air collision between me and a Twin Otter jump plane after departing ZZZ on my way home to ZZZ1. After clearing the Class B airspace of ZZZ2; I was instructed to climb to 13;000' and to intersect at ZZZZZ on my flight plan route. Shortly after intersecting; I was instructed to turn to a heading of 130 to avoid a parachute jump zone that had become active. So; I made a right-turn to that heading while still at 13;000' as instructed.After about a minute on that heading; my ADS-B traffic alerted me to 'traffic 12-o'clock same altitude less than 1 mile' at which time I saw the jump plane diving down directly in front of me after it had released its jumpers. It was no more than 50-100 FEET in front of me and in that instant I could read its tail number and the name of the jump school as it descended in front of me right off my nose. The plane (it was a Twin Otter) didn't see me as it passed in front of me because it was in a turning descent away from me as it spiraled down to the airport. I had no idea it was above me as I was supposedly under the watchful eye of ATC while on my IFR flight plan and on their vectored heading and ATC was talking to the jump plane and knew its position. However; ATC never gave me a position report on the Twin Otter and I never saw it or knew its proximity to me. Only when they became level and lower than me did they show up on my ADS-B receiver not before. So; I was oblivious that they were directly above me. It happened so fast that there wasn't time for evasive maneuvering as it was in a steep descent and was below me in an instant.A few seconds after that 'near-miss'; I called out on frequency to ATC and asked 'What just happened here? Because a plane nearly collided with me' (or something to that effect). No reply from ATC at all just crickets. They never acknowledged me again until telling me to change frequencies. I don't think the jump plane pilot knew my position any more than I knew his position because I did not hear a call-out of my position to them on frequency.