While playing around with bootstrap less files and css adjuncts. i came to the conclusion that glyphicons can not be loaded, even they are in the same adjunct folder:
There is my less & css file:
\target\classes\org\jenkins\ui\jsmodules\css\bootstrapprefix.less
same spot for the glyphicon font:
\target\classes\org\jenkins\ui\jsmodules\css\glyphicons-halflings-regular.woff2

I can load js or css files, but i cant use fonts like this. To debug this i used the @icon-font-path: "fonts/"; parameter.