This has not effected any previous versions to my knowledge and is quite the hassle. 
Explanation: Pigmen that spawn via random ticking portal blocks will most of the time TP straight through, causing random lag spikes as they reload dimensions, and entity process the other side. 
How It used to work: Pigmen that spawned in portals were given the portal cooldown effect on spawninig that prevented it from tp'ing through, and would have to wander away from 20 seconds and then wander back to tp through.
The old mechanic was much more understandable even if it did load the other side, it wasn't constant, and didn't break certain uses of portal pigmen, like portal pigman xp/gold farms.

Note: This issue has existed since 1.14.0