Usually, the documentation on the site is a little behind. In this case the documentation is actually ahead of the current version. This is something that has been added for 5.5, which hasn’t been released yet.
The first hint that this was the case was new “slide” on/off switches for the true false fields, which is another feature that’s in 5.5. I had a look into the code and confirmed that ACF checks the value returned from the filters.
In the current version there isn’t any check to see if the value that’s returned from your filter is false or is a field, and it still tries to display the field, which causes PHP errors.
