We've been experiencing some slightly bizzare behavoir on buttons for a while now. The text on load seems to be positioned to the bottom right of the button visual and then when you hover your mouse pointer over it, it repositions to where it should be (and where it shows on power bi desktop).
This only seems to be happening in embedded content as well. Screenshots below - first on load where the button text is offset and then second is in the process of hovering. We use buttons a lot of the time to help users drill down through axis on visuals.