Glossary entries have three fields: source term, target terrm, definition or note
However, glossaries might be monolingual, including only term and definition (a definition helps the trarnslator understand the term).
In those cases, the target term slot is empty (e.g. in a TSV file):
term\t\tdefinition
Terminology check (i.e. glossary adherence) understands the empty target term slot as an actual term and expects to find that empty string in the translation, but doesn't find it and raises the issue.

No terminology check runs (and no terminology issue is raised) for glossary entries that don't have a target term.
OMT package attached.
I try to write some codes to implement this feature. Here is the link for this pull request: https://github.com/omegat-org/omegat/pull/2023
Hi @feng, thank you so much. I have run OmegaT from branch fix/bugs-1315-empty-target-term-warning in your omegat fork and I cannot reproduce the bug. I don't have the technical insight or the authority to review your PR but it seems a fix for the issue I was reporting. Congrats!
Thank you @msoutopico for checking the code !