Michael, hopefully I can shed some light on it.
For Master Library dimensions and measures, the comments you add during load will not show up when you add a field or a calculation as a dimension or measure respectively. The master library takes its metadata from the information you enter into the name, description, and other fields like the tags entry to display when you click or mouse over those items in the Master Library.
It appears from the testing I've done that I can confirm what you are seeing is the same. If I add a comment to a field, and mouse over or click on the field in the fields section of the editor, the comment information does not show.
I recommend having a look at the Governed Metrics Service. This allows you to take your dimension and measure metadata from a centralized location and distribute it across multiple applications in a Qlik Sense site. The information you place in the table that would supply the Governed Metrics Service will load that information into a dimension and/or measure in the Master Library so that when users go to access that information they will see a description of the metric and can search for it based on tags you supply.
Please mark this reply helpful or answered if it does either one or both of those things.