No. They do not always return to reward inventory. It depends on the specific rules/conditions YM have set for each specific flight at the time of the cancel. As an obvious example say your flight is down to zero J seats for sale and someone cancels a J reward seat, it's almost certainly not going to go into U, but back into J inventory because they will want to try and sell that seat. OTOH, a flight with wide open inventory would probably return to reward inventory. My understanding from conversations with YM people at various airlines over the years is that these allocation rules can change for each flight sector/route etc depending on date and of course have constraints depending on the state of the flight in question.
So while it might often be observed the return to the same inventory from whence it came I would, in no way, guarantee this to be an absolute given or to be expected.