  Polarization measurements of gamma-ray burst (GRB) afterglows are a promising
means of probing the structure, geometry, and magnetic composition of
relativistic GRB jets. However, a precise treatment of instrumental calibration
is vital for a robust physical interpretation of polarization data, requiring
tests of and validations against potential instrumental systematics. We
illustrate this with ALMA Band 3 (97.5 GHz) observations of GRB 171205A taken
$\approx5.19$ days after the burst, where a detection of linear polarization
was recently claimed. We describe a series of tests for evaluating the
stability of polarization measurements with ALMA. Using these tests to
re-analyze and evaluate the archival ALMA data, we uncover systematics in the
polarization calibration at the $\approx0.09\%$ level. We derive a 3$\sigma$
upper limit on the linearly polarized intensity of $P<97.2~\mu$Jy,
corresponding to an upper limit on the linear fractional polarization of
$\Pi_{\rm L}<0.30\%$, in contrast to the previously claimed detection. Our
upper limit improves upon existing constraints on the intrinsic polarization of
GRB radio afterglows by a factor of 3. We discuss this measurement in the
context of constraints on the jet magnetic field geometry. We present a
compilation of polarization observations of GRB radio afterglows, and
demonstrate that a significant improvement in sensitivity is desirable for
eventually detecting signals polarized at the $\approx0.1\%$ level from typical
radio afterglows.
