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I think that might be related to bug CMS-32835 which should be fixed in CMS.Core 12.21 (hopefully release next week)
The bugfix was released yesterday EPiServer.CMS.Core 12.21.1 (optimizely.com)
You probably want to make sure you updated EPiServer.Cms.AspNetCore.HtmlHelpers to 12.21.1, not just the cms.core
We're using TechFellow.Optimizely.AdvancedContentArea for display options and up until recently it has been possible to read ""AdvancedContentArea__SelectedDisplayOption" from the ViewComponentContext.ViewData. After updating from 12.19.1 to 12.21.0 this has stopped working.
I saw there was a bugfix in the most recent update for the viewbag in blocks being empty, but this seems to be unrelated.
https://world.optimizely.com/support/bug-list/bug/CMS-32835
TechFellow.Optimizely.AdvancedContentArea has not been updated, so the issue is not here.