If the Selection Radii are changed from within a design that uses inch units, it causes a rapid spiral to them becoming zero whenever any changes to any of the options are made.
In spite of the statement “Radius dimensions are shown in screen units that do not depend on sheet units and the zoom factor” PTC customer service has verified with the programming team that they actually are metric units. If you look at them with a design sheet open that has inch units, they will be 1/25.4 of the radii you see without any sheets open.
If you change the values while in the sheet, the numerical value of the inch dimension will be transferred into the default value without converting it back to millimeters. The next time you open a design, the program reads the numerical value as millimeters and divides it by 25.4 to “convert” it to inches, making it.
If you change ANY option (not just the Selection radii), this process happens each time: assuming that the numerical inch values is millimeters and saving it as the option, thus dividing it by 25.4 each time.
The programming team has told the support team that this is working as designed and that you should never change the radii with a design sheet open.
It does appear that closing all design sheets and setting the radii will end the spiral, but this sounds like something was overlooked in the programming (going back several revisions) to not convert the inch units back to millimeters if it is changed with a design sheet open.
I suspect that most people have never seen this because there is little to no need to change the values. I have seen it happen because the values have mysteriously changed, causing issues selecting items with a design sheet open. The typical reaction at that point is to fix it (with the design still open) so that you can continue with your work. That causes the death spiral to get going.
The real fix (and expected according to the programming staff) is to save your work and close the design, set the Selection Radii, and then reopen your work.