What I thought the minumum meant was that once you lvl up (my case lvl 54) the minimum value means that your exp is locked to that minimum... yet I still went down a level today.

Am I wrong or is it a bug?

Of course you level down. Why wouldn't you?

If you lack the experience required to the level, you should lose that level.

I personally think that's right. Otherwise, everybody grows, and nobody weakens through being lazy - that's a lot less fun