I use the exact same USB-C PoE splitter with my C8. I have not had any issues whatsoever. I use a UniFi PoE switch.
I'm using this one and seems fine...
DSLRKIT Gigabit USB Type C Active PoE Splitter 48V to 5V IEEE802.3af Power Over Ethernet for Raspberry Pi 4 4B https://a.co/d/gRnrZem
Yup - that's my exact set up as well. I even went so far as to get some Z-Wave devices discovered and switched back to the splitter. The issues came back. I ordered a different brand to see if that will make the difference.
Have you tried a different patch cable?
How about a different port on the UniFi switch? Or possibly a different UniFi switch altogether? I wonder if the switch is not supplying enough power. 
Do you have a bunch of other PoE devices possibly using most of what the switch is capable of providing power-wise via PoE?
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In another thread of z-wave issues I posted your findings. So far four of us that are having issues are all using PoE splitters. And all four of us are removing them and going back to regular USB adapters to test. Still waiting to hear from others but there does seem to be some correlation. There are lots of folks using the same splitter who are NOT having issues but maybe we got a bad batch of splitters here.
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Pulling the splitter also removes the POE provider (e.g., switch) from the loop, which could be the real source of the problem rather than the splitter. If the regular USB adapter works, seems like you'd still want to find a way to rule out issues from the switch/POE adapter.
True but at least in my case I’m using ubiquiti gear that’s pretty solid and I’m powering other devices using a micro usb splitter just fine. However this is the only usb c device I have so…
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Just seems odd, aside from the "we got a bad batch" theory, that the same splitter might work fine for some, not for others.
Though so far 100% of the people reporting in on the other thread are all using the same splitter. A couple days will tell.
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For what it is worth, I was using the exact same splitter with a C->Micro adapter on the C-7 and had no issues with the Z-Wave.
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Yeah unfortunately not all my z-wave issues went away... I think it was the placebo effect.
I tried another "brand" splitter and it also exhibits the same Z-Wave comm issues. No telling if the insides are the same make, however.
https://www.amazon.com/gp/product/B09GM8FB3X
FWIW, I left the ethernet part of the splitter connected and powered the hub separately. The Z-Wave issues went away.
I didn't see where you said what the POE source is. If Unifi/Ubiquiti, some of their devices are 24 vdc and some are the current standard 48 vdc. Could that be the issue?
That's the one I just ordered. When it arrives I'll plug it into my C8 and see if any issues come up. I'm running mine of a single POE injector IIRC (or might be a small four-port POE switch, can't remember for sure and don't want to crawl under a desk to confirm at the moment. 
Anyway, interested to see if any Z-Wave issues arise for me using it - my Z-Wave has not had any problems on my C8 using my existing 2.4A splitter that I also use on my C7s:
https://www.amazon.com/gp/product/B07QPRVM51/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
I had another series of dropouts after I replaced the splitter with a brick. But yesterday I replaced the brick with another brick, stopped all automatic backups, and stopped Hubigraphs long term storage. So far so good. I guess it is possible that the brick I used the first time wasn't solid.
USW-Lite-16-PoE - the switch is on the other side of the house in what equates to probably just under 100ft of Cat-5E
Reported PoE Power Consumption / Budget: 12.9 W/45 W
Obviously something is quite sensitive on the C-8 as I (we??) had no problems with the same splitter (and C->Micro adapter) on the C-7
Well I just had another complete C8 z-wave failure. All devices stopped reporting at just past 1:20. On a brick, no PoE.
Holy ■■■■ dude! what a catch.. this helped me too. Did you find an adapter that eventually worked?
Good reading...
Nope, I gave up after the second one exhibited the same symptoms.
