What is the reason?
Is it possible to have orphan blocks in POS?
PS. I have been staking normally the past week, just noticed the instance a few mins ago.
Yes you get orphaned blocks in PoS.
It's an analogous situation to PoW: part of the network accepts one version of the chain, and your newly mined (and un-matured) block is on another version. Naturally, the network syncs up and your block is left orphaned.