  Fast radio burst (FRB) source 20180916B exhibits a 16.33-day periodicity in
its burst activity. It is as of yet unclear what proposed mechanism produces
the activity, but polarization information is a key diagnostic. Here, we report
on the polarization properties of 44 bursts from FRB 20180916B detected between
2018 December and 2021 December by CHIME/FRB, the FRB project on the Canadian
Hydrogen Intensity Mapping Experiment the Canadian Hydrogen Intensity Mapping
Experiment. In contrast to previous observations, we find significant
variations in the Faraday rotation measure (RM) of FRB 20180916B. Over the nine
month period 2021 April$-$2021 December we observe an apparent secular increase
in $\rm{RM}$ of $\sim 50 \; \rm{rad\, m^{-2}}$ (a fractional change of over
$40\%$) that is accompanied by a possible drift of the emitting band to lower
frequencies. This interval displays very little variation in the dispersion
measure ($\Delta \rm{DM}\lesssim 0.8\; \rm{pc\, cm^{-3}}$) which indicates that
the observed RM evolution is likely produced from coherent changes in the
Faraday-active medium's magnetic field. Burst-to-burst RM variations appear
unrelated to the activity cycle phase. The degree of linear polarization of our
burst sample ($\gtrsim 80\%$) is consistent with the negligible depolarization
expected for this source in the 400-800 MHz bandpass of CHIME. FRB 20180916B
joins other repeating FRBs in displaying substantial RM variations between
bursts. This is consistent with the notion that repeater progenitors may be
associated with young stellar populations by their preferential occupation of
dynamic magnetized environments commonly found in supernova remnants, pulsar
wind nebulae or near high mass stellar companions.
