I ran into this same error when building this dynamic workspace a year ago and writing to existing, unpopulated geodatabase (GDB) in File GDB format. The solution was to add my template File GDBs to the workspace as resources in the Schema Sources control in the FeatureWriter parameters. I have now modified the workspace to write optionally to Mobile GDB format through an additional FeatureWriter set for Mobile GDB format. I have the FeatureWriter set up identically to the File GDB version (which works fine), but I am getting that "Feature Type Definition 'ftype_name' was requested, but not defined" error. The following screenshot shows the parameter setup for the Mobile GDB FeatureWriter (FeatureWriter_8), and the Feature Information for the first feature to be written as it comes out of AttributeCreator_38. I am baffled. The Schema Sources items are correct; the fme_feature_type, fme_feature_type_name, and geodb_feature_dataset format attribute values are correct. The _out_dataset attribute value, which points to the target GDB for the feature, is also correct. I have checked it. The schema in the target GDB is correct, and the schema in the template GDBs (which provide the schema source) are correct. As I said, the File GDB FeatureWriter works fine, and the the new Mobile GDB FeatureWriter doesn't. I just can't see the problem. any help is appreciated greatly.
ADDITIONAL INFO (10/3/2023): Just found that the BridgeAnno feature class (file GDB) being translated (to mobile GDB) has a 1:3654 anno ref scale, while the destination has a 1:500 anno ref scale. Might the anno ref scale actually be taken into account as part of the feature type schema definition that is being retrieved from the template GDB in the Workspace Resources? (Again, writing to File GDB works, though, so I don't know why the annotation would be treated differently when writing to mobile GDB.)
ADDITIONAL INFO #2 (10/3/2023): I just tested the anno ref scale idea above by just recalculating geodb_text_ref_scale to 500 and then writing out to the mobile GDB: no effect, same error.
ADDITIONAL INFO #3 (10/4/2023): I tried a classic Writer instead of a FeatureWriter and got the same error; however, the logged messages are slightly different. The below screenshot appears to indicate that the reader in the workspace resources is extracting the schema from the incorrect source. The reader is seeing feature classes like "UtilityTunnel" and "UtilityPoint," which are clearly NOT mobile geodatabase feature classes but file geodatabase feature classes. I did try removing the worksapce resources and re-adding them but got the same error.