Logged a case with Esri tech support. This issue has been addressed in WAB DE 2.7 and 2.8. Only workaround provided for WAB DE 2.6 was to convert the field to type text but that doesn't seem viable as the operators then change. The support analyst mentioned a bug that could be related. The bug has a different error syntax but analyst mentioned it's possible the bug was logged before getting to the particular error I was seeing.
BUG-000092975, Using the Query widget on a date field in a service published to ArcGIS for Server from a SQL Server geodatabase results in the error, “Incorrect syntax near [date]”.
Thanks all for your input!
Andrew