I've decided to fix some of the flakiness of the owncloud VM by making sure that the DHCP server (pfsense) gives it a specific IP address. Since it's tied to the mac address of the USB adapter, though, and not the Hyper-V VM, the solution isn't ideal, but it'll at least work. The VM that I have onlyoffice in is set to tell the router to not DHCP it and just give it a different address.
This (hopefully) means that owncloud should basically always be accessible unless something very weird goes wrong, and onlyoffice is more likely to be flaky. Since onlyoffice isn't something that I necessarily need significant uptime for, I can live with this.
I feel like this is only an issue that's a result of me using Hyper-V rather than an OS more suited to containers like proxmox. I imagine other setups would be better about guest DHCP leases. I'd be more than happy to use the Windows default switch settings if they just kept the blasted IP addresses fixed!
update: I deleted the static mapping because it simply wasn't working. I'm starting to think that switching to a docker container is the better idea