The setting does not work when the range is left at its default setting.
Setting does not work when only one default setting is modified.

version: 2019.2.618.40

Hello, Steve,   

By default, RadRangeSelector has a range [20,70]: 

If you change only the StartRange, it is successfully serialized in the designer file:

Following the provided information, I was unable to replicate the incorrect behavior with the latest version of the Telerik UI for WinForms suite. Could you please upgrade and give another try of the project with the latest version? Alternatively, you can try setting the range at run time.

 I hope this information helps. If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik

Hello, Steve,   

If I adjust the start/end range in RadRangeSelector at design time and associate it with a RadChartView at design time as well, the range gets adjusted in correspondence with the chart.

Design time:

Please refer to the below screenshots demonstrating the InitializeComponents method and its workflow:

Initially, you have the default range [30,70]:

Associating a chart will adjust the range:

You manipulate only the StartRange at design time and it would affect only the start, but not the end:

Shown form:

I would recommend you either to update both start/end range at design time or associate the chart and specify the range at run time:

            this.radRangeSelector1.AssociatedControl = this.radChartView1;
            this.radRangeSelector1.StartRange = 10;
            this.radRangeSelector1.EndRange = 70;

Should you have further questions please let me know.

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik