Why would iCloud Private Relay break internet connection behind a network switch?

Please let me know if this community doesn't allow tech-support type of questions, I couldn't find a decently populated community on lemmy for this sort of question and I don't want to go crawling back to Reddit to ask.

Anyway, I have used iCloud Private Relay since it became available on WiFi with Macbook Pro. It works fine as expected, unless I enable my VPN, which is understandable.

Today, I bought a bunch of ethernet cables and a TP-Link semi-managed network switch (TL-SG108E, it has some configuration options but works out of the box as an unmanaged switch) with the intention of upgrading my network so things like the PS5, my home server, and my Macbook dock have wired connections.

Upon getting everything set up, I realized the Macbook was still using the WiFi, so I disabled WiFi, and lo' and behold, no internet. Strangely, I can ping everything on my network fine, and access all my self-hosted stuff. I just can't get out of my network. Figured it might be something with my Pihole. Or, worse case scenario, a dud switch.

Bunch of bullshitting later with no success, I, on a hunch, went into my iCloud settings and disabled private relay, and immediately I have internet access. Everything is working as it should.

So... am I missing something? Why would the sole change of plugging my Macbook into a switch via ethernet break private relay? For that matter - what is private relay doing that could possibly break internet access? If it works on WiFi, I can't figure out what switching to a wired connection, or the introduction of a switch, could do to cause it stop working.

Lastly, any advice to get it working again? Some configuration somewhere I'm overlooking?