I need to revisit this answer, that I find to be a limited work-around, not a proper solution.
The offending and unsupported type "mediumint" is not from the MS/SQL database, where data are sourced, so it has to be something that FME itself generates on the fly. Aka a bug.
The database table has a field named ID, which is an "int" (32 bit), and it uses an identity value.
This is how the schema feature sees the field:
I really cannot understand, why FME cannot transfer this to a table in a Geopackage. The very same schema and data feature are used to create a GeoJSON with any problems.
Please advise.
Using 2022.1