I love your question.
I believe that the resets of the worlds are like backups, so in this way, all invasions would be affected and their day count would be returned to the day of the backup. Using your example, if the server was reseted to the previous day's server save, then one day was lost. So if the cave bear should appear today, it will appear tomorrow.
I want to make it clear that we have no evidence of this server behavior.