Craig pointed out to me that as1-tu-203.stp conforms to a schema called CONFIGURATION_CONTROL_3D_DESIGN_ED2_MIM_LF.

The schema that is named ap203e2_mim_lf.exp in the SCL repo is Ap203_configuration_controlled_3d_design_of_mechanical_parts_and_assemblies_mim_lf. Apparently, SCL is not checking the schema name from a part 21 file. This schema lacks PRODUCT_CATEGORY_RELATIONSHIP, which is used in as1-tu-203.stp.

In as1_pe_203.stp, the entities are not in order. I assume this is what trips SCL up, as it complains that it can't find entity # 1.