Some hostile mobs are invisible. They can attack, but they do not make noise and cannot be hit. Disconnecting and reconnecting allows you to see them and attack them.
Sometimes, if a mob is invisible to one player, others can still see and attack the mob, but there have also been times where a mob is invisible to both me and another player or the entire server.
It affects all mobs, as I have seen an invisible baby sheep eat grass and upon relogging I was able to see the sheep and an invisible creeper exploding my friend.
This issue only appeared in the most recent update for bedrock 1.16.100 among many other issues.