What the hell is a mid relief. Sounds like the product of different die pressures and a micro variant specialist.
Since there is a high and a low relief, I think “mid” relief is a middle, kind of average, could also be called normal.
In my opinion those kinds of variants are minting errors, which should NOT have their own year lines, I think the numista guidelines say the same? I know, you have some high and low relief on US coins, where it's because of a changed die, but here it's more a matter of more or less pressure used at the minting.