As of the latest Lightroom Mobile updates, watermarks will not use the font "Monospace" if selected. It seems instead to fall back on "Source Sans Pro", despite the UI still claiming it's using "Monospace". I haven't tested with other fonts but surely Adobe can figure out if those are affected as well.
An additional older issue: Lightroom seems to try to sync the font selection in the watermark settings between Desktop and Mobile. Since "Source Sans Pro" is the only one of the fonts available on Mobile to likely also exist on Desktop, with perhaps the exception of "Roboto" (as the others are all generically named fonts like "Monospace", "Sans Serif", and "Serif"), and since none of the multitudes of fonts you have available to select from on Desktop are likely to exist on mobile either, this results in your font selection constantly resetting to "Source Sans Pro" whenever you use the other platform to select a font and export a photo with a watermark.