I have had this issue, too. I have completely lit up the area around my portal and even tried breaking it when I don't need it and re-lighting it later, but mobs still appear on the other side. My game doesn't load the fastest (MCPE), so the mobs are free to attack me for a few seconds. It happens very sparingly with no seeming pattern. I'm not sure how I would replicate it because of this.
Kadan Criddle
I have had this issue, too. I have completely lit up the area around my portal and even tried breaking it when I don't need it and re-lighting it later, but mobs still appear on the other side. My game doesn't load the fastest (MCPE), so the mobs are free to attack me for a few seconds. It happens very sparingly with no seeming pattern. I'm not sure how I would replicate it because of this.