See this for the deprecation of @import.
Is this an issue unrelated to this plugin?
https://sass-lang.com/blog/the-module-system-is-launched
The fact that @import cannot be used means that the file specified by @import cannot be read.
After that, the CSS written in the SASS syntax is output as plain text, and it is not compiled into a proper CSS.
What I’m trying to read with @import is a file that summarizes @mixins and constants, but it seems that the cause is that it becomes undefined because it can not be read.
No errors or warnings will be displayed when updating.
If you remove @import, it will work properly.