When I downgraded (in the project where I’m trying to use the custom library/activity) from UiPath.System.Activities 22.4.1 to 21.10.5 the error went away.
This was because the Range was not filled in for the Workbook Read Range inside the custom activity. Per the documentation, it should read the entire sheet - and when run directly it does. But when run from the library in another project, it doesn’t. I had to explicitly fill it in as “A1” for it to work correctly.