Flextensions Bug
While creating demo videos for setting up Flextensions from scratch, I updated the EMAIL_FROM variable in Environment Variables as intended in the description. However, when sending emails, it would look somewhat like this:

It wasn't until I fully deleted everything in the EMAIL_FROM cell, waited for GS to save, and then typed the same thing that it would properly update.

I recommend changing the variable to “TODO” like in other cells and updating the description next to it.
DataC88C also had the same issue which was fixed by deleting the cell contents and updating.