I purchased the full version with the belief that Mail Merge would function on Linux with my existing, voluminous data. This is not the case.
Windows mail merges work as expected. The field names are respected.
On Linux, field names become "FIELD_1", "FIELD_2".
This holds true for both .xlsx and .sqlite.
EDIT
I have worked around this for .xlsx by exporting to PlanMaker native format and to .dbf - both methods work and allow mail merge from .docx or from native TextMaker documents.

Ubuntu 20.04