REWRITTEN after deeper investigation:
I am updating our application from NG-Boostrap v11 with bootstrap 4 to NG-Boostrap v15 with Bootstrap 5 and popper. We are also updating Angular 15 to Angular 16. Needless to say there are/were a lot of breaking changes with the bootstrap update, but we are solving them except for the fact that all the Bootstrap variables are coming back as undefined.
All the correct scss files are imported correctly and everything seems to be correctly configured as we ran "ng add @ng-bootstrap/ng-bootstrap" to set it up, but the variables continue to come back as undefined.
I should also note that the css files are compiling without errors. The variables are undefined in the browser
Finally found the issue. We were using css-vars-ponyfill to support old browsers and it was rewriting the variables and causing the issue. Remove this and all is well. Leaving here in case anyone else runs into same problem.