I couldn't find much else on this and I think it could be a bug. If you base symbology on a custom field / expression and specify Scale Range, Labels will not honor the Scale Range as they would when not utilizing a custom expression.
In the attached Custom.png, symbology is set on a concatenation of RoadClass + SpeedLimit and a Scale Range was set for local roads. You can see in the attachment that a line for the Road Centelrine (RCL) is not drawn, but the Label for W STATE ST and W KILBOURN AV are still present.
In the attached TwoFields.png the same concept is completed by basing symbology off of two fields, and a Scale Range was again set. You can see in the attachment that neither a RCL feature or Label is drawn as the Scale Range criteria isn't met; however, both the feature and label are drawn once I zoom in far enough to satisfy the Scale Range (TwoFields2.png).