I just updated to 1.19 and I have encountered a major issue with creating runtime datasources. Something has changed in the coding path as my Custom Widget was working in 1.18. I don't see anything related to this documented in the Breaking Changes. The behavior I am getting is extremely odd. The layers created this way will first load as expected then unload themselves in Preview Mode. See screen recording:
Stranger still, this does not happen in the Build Mode. The layers do not automatically unload even when using Live View and I can switch between sets of layers without issue. The advanced datasource functionality is not available in the Map Layers Widget, but the Select Widget works as expected.

I am able to use Select and send the results to a Table Widget in Live View.

I have not done much troubleshooting yet. I just wanted to get this question out so the Dev Team has something to think about over the weekend.