We (a government organisation in Australia) are also experiencing this error in apps built using the web app builder. We are running ArcGIS Enterprise 10.8.1. We have also observed that feature layers and vector tile layers appear to deplete the available Web GL contexts.
The web map builder doesn't prompt us a warning when adding too many layers to a web map. Rather, when a user opens a Web App (built using the web app builder), some layers do not draw, and this error image appears at the top left of the Web App's map frame.
This error image is often obscured by the Web App's zoom in/out buttons. However, if you click and drag the map, the error image moves with the map, allowing you to bring it out from underneath the zoom buttons.
We have also noticed that this warning message appears in the browser's F12 console screen:
This is causing some real headaches with our vector basemaps and feature layers not displaying correctly. Is anyone aware of a time frame for the long term solution @MikeMinami mentioned regarding rolling out Java Script 4.x API to ArcGIS Enterprise?
Many thanks,
David