Problem/Motivation
Currently, stubs are blindly created for any referenced entity ID on the source site, even if that is a non-existing entity reference ID!
If you're also validating all your migrated entities (like you should be), which is possible since #2745797: Add option to content entity destinations for validation (see https://www.drupal.org/node/3073707 for the CR), then all of those rows will also not trigger validation errors, when in reality they should.
This means you end up getting a false sense of security: your entities are migrated and pass validation.
Proposed resolution
Only create stubs if the source plugin finds a row for the given source ID values.
Remaining tasks
Test coverage.
User interface changes
None.
API changes
TBD
Data model changes
TBD
Release notes snippet
TBD