Since the 1.15.1 update when I go through a nether portal there has been a creeper/zombie/skeleton within the portal at least 80% of the time. This is happening during the day and as if a mob came through the portal with you as if was night time. Again this is only happening when going through a nether portal ENTERING the nether.
Entities can travel through portals. Probably they just wandered into it at night time.