The scenario goes like this:
print/texlive-full is configured with DOCS and SOURCE both turned on.
make run-depends-list contains print/texlive-texmf-source and 22 additional dependencies.
ports-mgmt/synth compares the recorded runtime dependencies of the generated package against the port's runtime dependencies, and surprisingly finds a discrepancy.
print/texlive-texmf-source is missing from the recorded runtime dependencies.
Thus, the package is rejected and deleted, which cascades to local metaports depending on print/texlive-full.
Deselecting SOURCE makes synth happy.
I know the defaults are DOCS on and SOURCE off, but I think Mk/bsd.tex.mk should handle the above. Feel free to disagree.