Height calculation of blink while being underground

I don't know if it may occur in various caves but in Undercity it's very consistent. And if it can happen in Undercity, then there is the possibilty that it happens somewhere else too.

When you use the mage spell 'blink' in the outer ring of Undercity (like the quarters) or the passage ways, which connects the inner and the outer ring, then you get teleported to the surface of Lordaeron (above Undercity).
The reason why this is happening only in the outer ring is because above the innder ring is now terrain, just a hole in the map.

At first I suspected vmaps to be the culprit. But even after disabling height calculation with vmaps (vmap.enableHeight = 0) in the config I got the same result.

I'm not sure if the calculation in the spell effect is entirely to blame or if there are different systems involved which causes this behavior.

I'm certain this did not happen in Rel18. But don't know about Rel19, Rel20.