I’m playing on the PC and have two mercenaries in my training grounds (Velen and Murk-Eye), I clicked on the earning XP button and they gained their levels from having left them in for awhile, but when I went to click on it again to take them out, it just popped their portrait up, and I believe gave them a small amount of XP. I know this can happen if you don’t click the button again after earing the XP and I did that, and the earning XP button came up with no confirmation of being able to remove them. It doesn’t matter how often I clock the earn XP button, it will not give me the option to remove them. I reset my game twice already, I tried clicking on their portraits, I tried dragging other mercenaries over (which I know doesn’t work because it will say the grounds are full, which it did), nothing worked. They are stuck in the training grounds.
EDIT: I was trying to still get them out, by moving another mercenary onto them and now their card visual (like when they earn XP) is stuck floating above the opposite mercenary. Hovering over them, highlights it like you can click them, but obscures the UI underneath. Leaving the training grounds and coming back did not remove them, but leaving the mercenaries mode did. I could not repeat this bug after coming back, just figured you should be aware of it.