Backgrounds in labels, especially for composite labels, would be very helpful for field users. Currently, this appears to be functional in ArcGIS Pro, but the settings for the <BGE> tag and the <part> tag do not seem to carry over to Field Maps in a mobile map package. I've included examples below.
What a symbol/label looks like in ArcGIS Pro (Some data is blacked out for privacy)
This uses the following expression:
"<FNT name='Century Gothic' style='Regular' size='8'><BGD red='230' green='230' blue='115' width='.25' alpha='50'> <BSE>" + $feature.esPhasing + "</BSE> </BGD><BGD black='0' alpha='50'> " + $feature.esMeterNumber + " </BGD></FNT>"
Meanwhile, in an MMPK uploaded to AGOL and opened in Field Maps:
The background information is gone, but the rest of the expression appears to evaluate fine. The effect is similar using the <part> tag and composite callouts. There are workarounds for displaying composite information in labels of course, but this added functionality would help with clarity and organization of the information. Also, a final note: I know the MMPK retains the information because I imported it back into Pro, and it displayed fine.