I took some time to investigate this issue and working workaround seems to be adding .po and .mo files for default source language even if the files are empty.
E.g. my site default and source language is en_US for which I don’t need po/mo files, because language strings are hardcoded. Another language is de_DE for which I’ve got po/mo files generated and translated. Adding po/mo files for en_US language seems to solve the issue.