Obviously, there is no node_modules directory inside assets and this is a installation of sage with no CSS framework. Also, these CSS selectors live in _global.scss.
The node_modules directory it is referring to is the one in your theme root directory. Check if bootstrap is installed there, if not then I do not know why it is referring to a bootstrap file. That a { selector from the picture is also added in bootstraps _reboot.scss so it looks like it might still be included.
Yes, Bootstarp is inside root node_modules. But this is a Sage installation without Bootstrap and without any other CSS framework. Is the Bootstrap CSS reset (_reboot.css) included regardless of whether Bootstrap is installed?
No, this a { selector belongs to _global.scss:
Maybe, can be that bug you mention. Is not possible for me to know it for sure (I’m not a developer and these are my first steps with webpack and Sage 9. All seems very dark yet. Too much ignorance, I’m afraid ). Anyway, if that is the case, why it is affecting only to me?
Ok! I forgot that this Sage installation was done cloning git repo (in order to work with subtrees), not with the installer. So, Bootstrap was not removed. I’ll try the installer way.
[EDIT] Installing Sage from composer installer works without map errors.