Pycharm warns package requirement not satisfied when using pipenv to install package

The solution is in this answer on the JetBrains support forums.

The steps are as follows:

1. Go to File > Invalidate Caches/Restart... and press Invalidate and Restart.
2. Once restarted, add the interpreter back, wait for the stubs to be rebuilt and then check to if the problem has been resolved.
3. If not, then first remove the pipenv interpreter as the project interpreter by going to File > Settings > Project: > Project Interpreter and setting the project interpreter to No Interpreter. Click Apply and OK. Then repeat steps 1 and 2 and see if it solves the problem.

I'm not sure why this problem occurs, but it seems to solve it when it comes up.

EDIT 07/29/19:

This bug has been fixed in the 2019.2 Release of PyCharm.