residential plywood roof deck: does the long edge have to rest on a purlin?
There are little metal joining clips that are used for light duty application of roof decking to rafters on 24" centers without lathe. If this is allowed when there is no lathe, then surely it would be sufficient if there is lathe.

The plywood is stiffer in one direction (I think the long direction) and it may be that the stiffness along with the joining clips (if used on the long side) are sufficient. One side of the plywood (I think the short side) is more flexible and that is what needs support by joining it on the rafters.

Editing If I could do it, I would get extra lathe and support the joints with lathe. How much better will you feel for all the time you'll be under that roof to have the joints fully supported?

More This roofer uses plywood strips to connect the long sides of the decking sheets: https://www.youtube.com/watch?v=Ij00l1PzgU4 But I don't think his strips are wide enough. I would think 2" min, 3" best.

He says in this video he uses 1 x 2" battens and supports the long edge. The batten or lathe supports the edge by tension. https://www.youtube.com/watch?v=WwsHolj_bnc