After some discussion with mark.benvenuto and kevin.albertson, it appears the windows_test_build_and_upload task ((https://evergreen.mongodb.com/task_log_raw/libmongocrypt_windows_test_build_and_test_and_upload_92183bcb350ac5d60c87b89628e23e599daf435b_19_10_28_19_19_18/0?type=T#L868 ) in EG appears to be creating mongocrypt.dll in debug mode, as evidenced by the presence of the following flags: Od /Ob0 /MDd.
We should probably be targeting RelWithDebugInfo