If it's a bug in the core code it would make sense that it's happening regardless of whether a skin is being used or not.
The fact that it's only happening to the player portrait also makes sense. After all, that's the only 3D element that is on screen all the time. You can't maintain a target through zoning, so the target portrait isn't being affected because it's not present upon zone-in.
You said that for some people it's getting bigger and some smaller. This also fits the theory. With my guy it turned out that he had scaled the vitals larger and he was experiencing the portrait growing. I bet if you check with the ones for whom it was shrinking, it will turn out that they'd scaled the vitals down. If the scaling is being applied cumulatively with every zone-in, you would naturally see the portrait slowly shifting in the direction that the UI scale has set.
My person tested and discovered that it happens regardless of whether global UI scale is applied or just the vitals. The only thing really remaining to test is whether it happens when a scale amount is used that would result in a whole number rather than a fractional value. For example, 75% scale rather than 80%. If it happens with either value, then the engine just doesn't like scaled vitals. If it only happens with the latter value, then it comes down to fractional values.