Answering to myself.
As a workaround, I used a "blank button" - a button with a transparent image and I set a tool tip to that, it works just fine for what I want.
In a more general way, I am in the process of removing text labels and text from all my knobs because they exhibit weird scaling (often cut with ...)
Sometimes the font weight is slightly off, I have to reload some presets a few times before it gets back in order so I think I will stick to write text, glyphs and symbols on the background image from now on.