I haven't found a solution yet. It seems that upgrading a widget's configuration type is a breaking change. Hopefully an Apple engineer can confirm and/or provide a solution.
mpmontanez
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It seems like iOS 14 tries to show a cached version of the widget while it initializes an updated version, and if the widget fails to initialize this flickering will occur. I reduced my widget's snapshot logic to use hardcoded values to optimize its launch performance. Seems to have helped a bit, but it's hard to tell if that actually solves the issue.