Enabling 'Labels on a Multipoint Geometry' layer in a web map disables the redrawing of symbols when zooming, and the labels are not displayed.

Workaround

1. Create a multipoint feature class in ArcMap or ArcGIS Pro
2. Publish it as a Map Service to an ArcGIS Server
3. Add the layer to a web map (ArcGIS Online or Portal) using the using layer URL
4. In the content pane of the Web Map select the More Options > Create Labels > Ok for the multipoint layer

Steps to Reproduce