Gantt App / group by linked record

I’m running into an issue trying to “group by” a linked record. The grouping works as expected when I use one linked record field, but it does not when I use another.

Video here - https://www.loom.com/share/219844a0a5244d7c9760eccf9a6465ad

One difference is that the Contractor field is linked directly. The Project field (the one I want to use that does not work) is a Lookup field.

If I create a simple Formula field that is only “=[Project]” to get it to display as text, then I can group by that formula field as intended.

Has anyone run into this?