Why is the word "CONCAT" displaying in labels on my map?

[AllenDailey1]

I'm having a weird problem with labels in ArcGIS Online map viewer.

I published a map service from ArcGIS Pro to our standalone ArcGIS Server. Then a web map in ArcGIS Online contains layers from the map service.  

I configured the layers' labels in Pro (Pro version 3.3).  I used a Python expression for the labels:

Many months ago when I first published the service, the labels worked fine in ArcGIS Online. But I happened to discover today that now the labels show up in Map Viewer as the literal text contained my Python expression, but with the "+" replaced by the word "CONCAT".  

Label should be: District 1
Label shows up as:  "District" CONCAT 1

Why???  

Any ideas of what to do about this?  I would really prefer to configure labels in Pro rather than the AGOL Map Viewer, so that I don't risk labels getting wiped out when I republish the map service.

Thanks for reading!

[DanPatterson · Accepted solution]

well, esri says use arcade

BUG-000158454 for ArcGIS Pro

because it won't be addressed

[AllenDailey1]

Thank you so much for sharing this!  I had tried a couple different google searches on this topic, and I searched Esri Community, but I never found info about this bug.  Thanks.