I'm quite baffled.
I had a Marantz sr6011 receiver. I set it up to receive its IP address through DHCP in the normal, boring, old-fashioned way. I went to use the smartphone app to control my zone 2 input, and it could not find my receiver. I went to the receiver and checked its settings- it has an IP very similar to my public IP (74.xxx.xxx.xxx as opposed to my internal addressing of 10.xxx.xxx.50-254...and it's a single subnet internally), but I'm a bit too paranoid to list out the rest), but it is definitely different than my router's public IP.

This is the SECOND time it has done this in the last 2 weeks- once connected directly to the router, the other explained below in detail through a switch.

I have a fiber-to-the-home connection, it's terminated in my house as an rj45-ended cat5e cable. There is no modem/router in the house/provided by the ISP (LUS Fiber).

Router is a MikroTik Routerboard hEX PoE RB960PGS. It's got a fairly simple setup- nothing elaborate. There is a hole on the firewall that's routed to a static address on one port for another computer that was setup well after the first time this issue occurred.

The whole setup from outside is:
Outside cable from ISP-->female to female rj45 coupler in patch panel (for aesthetics, this also happened plugged directly into the router)-->small patch cable-->router eth1 (WAN)-->Router eth2 (LAN/PoE)-->cat6a cable into Ubiquiti UAP AC Pro--> Cat6a into LAN port of AP--> another rj45 female-female coupler in a patch panel -->patch cable--> port 24 of a Linksys LGS124 switch. Port 23 of the switch goes to a short patch cable coupled to the cat6a connector going to the receiver.

The only "unusual" things you'll see above:
1) I'm using a patch panel with couplers for a cleaner look and to reduce the strain on the cables (switch has ports on the front but my cables are run in a cabinet behind it), but this was an issue when it was plugged directly into the router as well (specifically into eth5, not that it matters)
2) The switch is fed through the LAN port of the AP instead of directly by the router. Speedtest suggests 930mbps+ using this method on a handful of other hardwired clients, and the issue occurred, again, whether it was plugged into the router or not.

On my DHCP server page on the router, I think all my other devices are present with the exception of 2 static routes I've got on two desktop computers that are outside the scope of the DHCP server.

One last semi-interesting bit: The marantz receiver has a webpage when you type in its IP address. when I typed in the "public" IP from inside my network, I could change the settings on the receiver (with a different outward-facing IP than my own router's from inside my network). I, in a panic, attempted to reach it from outside my network (my phone), but it couldn't find it. So my phone couldn't see it over my WiFi network from the app, could by using its IP address, but couldn't by using its IP address over the internet.

The router has a "make route static" option on the client-lease table. Once I reset the Marantz network (which is an unusual interface...you can't edit the IP address unless it HAS network connectivity, which is a pain if you, say, forget to reset the subnet mask from an external DHCP server to your own while trying to manually pull the receiver back into your network), it now has its own lease INSIDE the scope of the DHCP server, but reserved. So hopefully that works.

But has anyone ever had a device in your network pull from your ISP's DHCP server instead of your own? What exactly is the "hole" it's going through, and how can I patch it/prevent this from happening in the future? It's hard to find anything online of anyone having a similar issue.