I agree with you, I was surprised that it did not require a factory reset.
And for my case I think the mess I had was due to changing from OpenGapps to MindTheGapps simultaneously. Had I reflashed latest 18.1 with MindTheGapps before the upgrade to 19, it might have worked smoothly.
(OpenGapps is not yet available for Android 12 / hence I was forced to do the swap. I Prefer OpenGapps, as they have lesser packages than MindTheGapps, as I mapped out extending the OpenGapps compare schema last year)