Revised Question (based on the now documented AVPlayer bug):
I found a workaround using this code as a base: https://stackoverflow.com/a/25623647/6697755
I found isWWAN flag can be used to check network type
var flags: SCNetworkReachabilityFlags = []
let isCellular = flags.contains(.isWWAN)
It seems to be a pretty reliable check. So I can now derive a network type change and reboot the player.
...only took me about 80 hours to find this workaround ;|