Update: reverting my VS2019 v16.9 install to v16.7 fixed the issue.
Just noticed this note: https://marketplace.visualstudio.com/items?itemName=SSIS.SqlServerIntegrationServicesProjects
Common Issues
There is a regression in VS 2019 16.9 that will cause script task/component not work properly when editing script. Please do NOT upgrade to 16.9 currently. If you have already upgraded to 16.9, please rollback to 16.8 or earlier version if you are using script task/component. VS team is working on the fix. Sorry for any inconvenience.
I'm uninstalling v16.9 right now, then installing v16.7 (v16.8 isn't available in my VSS list).
Will report back.