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).
Hmmm... I do see the behavior you are talking about. When you use a custom expression to set the symbology values and you set the scale for these symbols using the sliders, the labels for these features draw regardless of the scale you are viewing the data at in your map, and this is not true when you just use two fields rather than a custom setting (in this scenario the labels turn on and off with the lines).
I would say if its a bug its a small one...maybe an inconsistency.
You can fix this behavior by setting the scale at which your labels turn on and off (as in the image below). When I set the In Beyond or Out Beyond values for both layers in the table of contents (one set up each way you describe), the labeling functions as expect (and identically) for both scenarios.
@KimGarbade I already had that set for the one Label Class that the layer and the entire project is currently utilizing. I'd have to go in and create a separate Label Class for each symbology level in the layer for it to honor a similar scaling effect. This was my plan anyway, as I believe it is a more efficient approach when it comes to rendering maps, but I hadn't gotten to it yet and is why I noticed the apparent bug.
Like you said, it does appear to be a minor bug; however, I figured I'd throw it out into the universe in case it has any wider implications.