Missing members but still loading data

Content:

We try to load data using file as a source. Whenever members are unrecognized we receive an 3303 error and the data load fails. Sometimes during other integrations the data load is partial successful (without error 3303) and the logfile specifies that certain records are loaded and some are rejected (I'm not talking about the transformation, but about the export to target). The whole load still throws an error in the process details page, but partial data is loaded.

Why does is this behavior different between data loads and what causes it?

Version PBC:

23.08.60