A family of Microsoft word processing software products for creating web, email, and print documents.
The rules are that the name must start with a letter (or an underscore, which makes it a hidden bookmark), is limited to 40 characters, and cannot contain any spaces.
Your example, "supp_fig_monocyte_cartoon", is a valid name, as I verified by creating that bookmark to surround a paragraph in a document named doc1.docx.
In a separate document, I created an INCLUDETEXT field pointing to the doc1.docx document, and inserted that bookmark name:
Updating that field displayed the paragraph correctly.
When you tried this, did you see an error message? If you did, what did it say? If you didn't, then what happened instead?
Also, how did you create the field, and what does its code look like?