If a portal has been created that hasn't had an exit portal generated by the player yet, and an entity goes through it - first of all it creates a spike in MSPT by several hundred, (MC-152635 and MC-117914) - and the entity cannot travel through.
The problem starts when the player travels through and loads up an exit portal and then travels back again. When the SAME entity is pushed through the portal again it once again causes a lag spike but the entity will not go through, even though an exit portal has been generated. Other entities who have not been pushed through the portal before the exit portal has generated will be able to pass into the other dimension. I have waited the cooldown time, tried breaking the nether portal, and even re-logging but the offending entity will not pass through.