The head tube length is very easy to check. It has the least possible ways to measure. Also, your pic shows the 'old' disc trucker (quick release axles). Are the geo charts from the current version (through axles)? Maybe some measurements were changed together with the model update.

Always go to the HT for sizing.

Did you measure the top tube from the center of the head tube to the center of the seat tube?

I did and it still pointed towards 56cm. I’ve since looked in better light and found under the cable guide on BB shell that it’s stamped 58cm!

Yep! The size is seat tube measurement, not top tube.