The report is not using classifications of the variable in question, and the debugger shows that the value is being set correctly in the call. Because we're not seeing a similar issue in Ad Hoc or Analysis Workspace, this seems to be something that is specific to Report Builder.
The issue started over the holiday break, and we had not updated Report Builder since 5.6.29.0. The issue persists after we updated to 5.6.35.0. No changes were made to the Report Builder settings either.

Sounds like I need to contact customer support.