Fixed in 2021.1, please see https://youtrack.jetbrains.com/issue/IDEA-135546
What was that?
All IDEs based on IntelliJ platform have a global setting telling whether to treat file systems as case-sensitive. The setting depends on OS: by default, Mac OS X and Windows are case-insensitive, Linux and friends are case-sensitive.
You can either move your projects, or tell an IDE to change its mind.
For the latter add the following line to idea.properties file (see this article for Help | Edit Custom Properties, see "Configuring platform properties" for more details):