I have found that when I have backgrounds enabled on labels they show up correctly in Map Viewer, but when viewed in the Field Maps app the text colour changes to white. If the background is white, and the text is white, the label is not visible anymore.
The easy workaround for this is not to use backgrounds, but I was curious if this happens to anyone else or just me?
I think you may have discovered a defect, if possible can you please raise a support case with Technical support with your full workflow so they can attempt to replicate and hopefully log a defect for.
But you are correct with the workaround, is not to use backgrounds for labels when utilising Field Maps.