I'm using the latest Ornatrix version 2.3.9 that just got released a few days ago with Arnold 4.5.1 on C4D r25 and 2023 for the Mac. The hair doesn't render at all in the IPR unless I add a Curves from Strands or a Mesh from Strands operator to the stack. Sometimes though, even with the Curves from Strands operator included, the hair doesn't render in the Arnold IPR; so when that happens, I've learned to turn off the Hair from Guides operator, save the file, restart the C4D app, open the file again, turn the Hair from Guides operator back on and then it renders again on the IPR. And yes, I have the Hair Render turned on in my C4D Render Settings too the whole time. When that doesn't work, I go as far as clean out the System and User caches and restart my Mac in order to get it to start rendering in the IPR again. Very time consuming process.