Same situation with me and does appear it would be an easy fix with a big benefit for Insightly users.
Alternatively if Insightly data was available in the desktop I could try do it myself, but this isn't possible at the moment. Insightly confirmed they have nothing to do with data definitions:
Thanks for reaching out.
Sorry that that's happening. Insightly doesn't have control over how Power BI reads our data, we just allow them to use our API to pull your data. The integration to Insightly was built and is managed by Microsoft.
I'd suggest reaching out to in the Power BI Community. Another user or Microsoft support member may be able to add their input or may have a solution.
If you need further assistance regarding your Insightly account, please let us know.
Regards,
Erin T.
Insightly Customer Service