Automigration fails for builtin plugins

The --debug flag causes direct abortion after the first attempted automigration:

1 monitoring@monitoring ~ % cmk --debug -vvR                                                                                                                                                                                               :(
Files: ([], None)
Traceback (most recent call last):
  File "/omd/sites/monitoring/lib/python3/cmk/base/config.py", line 1986, in _extract_agent_and_snmp_sections
    create_snmp_section_plugin_from_legacy(
  File "/omd/sites/monitoring/lib/python3/cmk/base/api/agent_based/register/section_plugins_legacy/__init__.py", line 235, in create_snmp_section_plugin_from_legacy
    detect_spec = create_detect_spec(
  File "/omd/sites/monitoring/lib/python3/cmk/base/api/agent_based/register/section_plugins_legacy/convert_scan_functions.py", line 397, in create_detect_spec
    _compute_detect_spec(
  File "/omd/sites/monitoring/lib/python3/cmk/base/api/agent_based/register/section_plugins_legacy/convert_scan_functions.py", line 365, in _compute_detect_spec
    scan_func_ast = _get_scan_function_ast(section_name, scan_function, fallback_files)
  File "/omd/sites/monitoring/lib/python3/cmk/base/api/agent_based/register/section_plugins_legacy/convert_scan_functions.py", line 104, in _get_scan_function_ast
    assert source != "", "Files: %r" % ((read_files, src_file_name),)
AssertionError: Files: ([], None)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/omd/sites/monitoring/bin/cmk", line 79, in <module>
    errors = config.load_all_agent_based_plugins(check_api.get_check_api_context)
  File "/omd/sites/monitoring/lib/python3/cmk/base/config.py", line 1415, in load_all_agent_based_plugins
    errors.extend(load_checks(get_check_api_context, filelist))
  File "/omd/sites/monitoring/lib/python3/cmk/base/config.py", line 1563, in load_checks
    errors = (_extract_agent_and_snmp_sections(validate_creation_kwargs=did_compile) +
  File "/omd/sites/monitoring/lib/python3/cmk/base/config.py", line 2005, in _extract_agent_and_snmp_sections
    raise MKGeneralException(exc) from exc
cmk.utils.exceptions.MKGeneralException: Files: ([], None)
1 monitoring@monitoring ~ %  

This is the case regardless of whether there are contents in ~/local/share/check_mk/checks or not.
The folder contains a few checks, but they seem to be unrelated to the error message:
monitoring@monitoring ~ % ls ~/local/share/check_mk/checks
agdsn_ladon_temperature bacula_jobs mikrotik_signal_sixty mikrotik_signal_sixty_remote mikrotik_signal_sixty_remote_stats

I’m having trouble sharing the -vvR output without --debug, because it’s a ton of output and the migration error messages for whatever reason are not carried along pipes like | tee cmk.log. Investigating that.