Steve, great investigation! It is the current mis-function of the LESS engine in the current branch. We are going to make it allow to add, but not override in next major versions. It requires 5.4 release.
However, with XC5 we track much more upgrades, even with custom mods it just costs far less for maintenance. As I said at the meet-up 62% of XC5 customers upgrade to the new versions. XC4 - only 4%. So it means the upgrading idea works, but need to be better and better every release.
Here is the ticket to fix the issue:
https://bt.x-cart.com/view.php?id=48739