@RobertChaney
Thanks for submitting a technical support case. I was not able to reproduce the issue, so maybe it's data specific?!
In any case, here are a few ways to calculate the number of vertices of a polygon:
1. Using the geoprocessing tool Calculate Geometry Attribute
2. Using Arcade to extract the Geometry ($feature) and count the number of points in every 'path'. You can use this to calculate a field or as an Attribute Rule validation.
3. Using a Python script you can probably create an 'in_memory' layer and not have to change your schema.
For method 1 you will have to add a field to write the number of vertices, then run Select By Attributes to find the features with 1 vertex.