In order for the browser to use LESS files, they must first be compiled into CSS. It is possible that this compilation process is not taking place in this scenario. Despite the similarities between LESS and CSS, and the leniency of browsers in handling malformed CSS, some styles may still display correctly even if the LESS has not been compiled.
To verify the compilation process, you can use tools such as Fiddler or browser-based debugging tools (such as pressing F12 in Internet Explorer, navigating to the Network tab, starting capture, examining the response body for the .css file of interest, and ensuring it is valid CSS and not .less).
Additionally, ensure that you are not viewing a cached version of the CSS when accessing the site through the VS debug server. A quick way to check this is by enabling "Private browsing mode" in your browser, which will not utilize any cached resources.