According to TAC, this is a hardware limitation of the c9300X-48HXN-E and we can only use 6 of the 8 ports on the 9300X-NM-8Y.
From TAC: "
About your issue, unfortunately we are facing a limitation - it is an expected behavior. Exactly the same ports you want to use are disabled in this switch in the version the switch is using. For now, there is no way to enable them. If you need more information, visit the next link: https://bst.cisco.com/bugsearch/bug/CSCwi63462
As workaround if you need to use 17.6.5 version but you will not be able to use appgig interfaces."
From the bug report:"Symptom: C9300X-48HXN was introduced in 17.07.01, due to a hardware limitation the last 4 ports (5th, 6th, 7th, and 8th ) were disabled initially. Starting from 17.08.01a onwards, the last 2 ports (7th and 8th port) are disabled when using C9300X-48HXN.
Conditions: C9300X-48HXN with C9300X-NM-8Y installed.
Workaround: The behavior is related to a hardware limitation thus the ports cannot be enabled."
In our scenario, we don't need to use all 8 ports on either switch, but I wanted to find out why I only saw 6 of them active before I put them into production. If we did need to use all 8 ports, the TAC engineer said the only workaround is to install a much older OS from before this switch model was officially supported, which doesn't sound like a good option to me. They must have disabled those ports for a reason, right? My boss isn't happy with this answer, as it limits our future expandability with these switches. The ones I'm working with now are only the first out of a whole batch of them that we bought to replace switches at several of our buildings. It's frustrating that this particular limitation doesn't seem to be documented anywhere except in Cisco's bug report system, and our reseller/partner had no idea about it, either.
