I think it might be because there are still people on the older versions of the wallet, which was PoW/PoS, and that is somehow disrupting the network in someway (this is just a suspicion of mine, I have no idea if that is even possible). I am still seeing this happen on my v1.7 wallet from time to time as well, even though I re-downloaded the entire blockchain (or at least I think I did), but it hasn't affected my ability to mint and send/receive transactions.