Responses
This manga explains that he is a special case where the mark didn't disappear when his alpha died.
And every author's omegaverse is slightly different including matters like being able to remove a pair/bond or not.
If Sei didn't love his mate, then why were they mated? Doesn't he realize that the mark stays there forever? Did he possibly think that he'll slowly fall for him within time or smthn?
Also, in some omegaverse, the alpha can make the mark disappear or when the alpha dies.