All buttons no matter what Addon they are are overriding the font-size with 16px. These are not in the SP Page Builder style files but are generated by the addons themselves in the render.
On a website, buttons of sizes sm, md, lg, xlg are used and therefore each button has different font sizes, but this bug is making all of them, regardless of size, have 16px. If the fontsize field is empty, you should leave it that way and not put a supposed value.
It does this on buttons created with SPPB 3.x and which don't have a font-size set because they didn't have a field for that.
If a button doesn't have the font-size set, they should leave it ungenerated instead of adding 16px.