I doubt it's these lines specifically that there are having issues, but rather one major issue when moving between LESS and SASS: LESS basically allows any named class to be used as a mixin, SASS does not. However, SASS allows you to "extend" an existing class. This is semantically very similar (a new class takes on the characteristics of an earlier defined class) but it's implemented in a different and quite possibly non-compatible way.

This isn't a problem for anything in our own LESS - we can just rewrite things we want to use as mixins as proper SASS mixins. It becomes a problem when using the SASS version of Bootstrap - several Bootstrap classes were used as mixins in the original LESS version, and I can't duplicate that in the new version without copying over the Bootstrap classes, which seems to me to defeat the purpose of using Bootstrap as an external library.

Hopefully the issues can be resolved without directly copying in Bootstrap CSS, but it's entirely possible that what you're seeing is a slight change in rule priority order caused by the change from LESS mixins to SASS extends.