Files added as Resource are removed from a class library project

**Visual Studio Version**:
16.10.3

**Summary**:
If I try to set a file as a Resource, the file is removed from project (same as right-clicking and clicking to exclude from project) and can't be added anymore.

**Steps to Reproduce**:

1. Create Class Library project (.NET 5).
2. Create folder (I named it 'Resources')
3. Paste a file (could be a .gif or a .ttf, or whatever).
4. Try setting that file (or multiple files as Resource).

**Expected Behavior**:
File is set as resource or warning appears telling me that I'm missing some dependencies or that it's not possible to add resources in a class library.

**Actual Behavior**:
File is excluded from the project and can't be added back unless the folder is renamed or the tag `` is removed from the csproject.

**User Impact**:
?