UniFi Chromecast discovery issue across VLANs when connected to downstream switch

Hi all,

My Chromecast is on the IoT VLAN, has internet access, and responds to pings from Private. However, it doesn't appear as a casting device in YouTube when my phone is on Private.

I'm running the following setup:

• Gateway: Cloud Gateway Ultra
• Main switch: USW Flex 2.5G 8 PoE (STP priority 0)
• Downstream switch: USW Flex 2.5G 5 (STP priority 4096)
• APs: 3 connected to the main switch
• Chromecast: Wired to the downstream 5-port switch

Both switches are on a management VLAN, with all other VLANs trunked between them. I've allowed Private -> IoT, allowing return traffic so I can stream and connect to IoT devices. mDNS is set to auto, IGMP Snooping is enabled on Private and IoT, multicast is set to Auto.

• Chromecast wired to the downstream 5-port switch -> Not discoverable from Private.
• Chromecast wired directly to the main 8-port switch -> I can cast YouTube
• Chromecast connected over Wi-Fi -> I can cast YouTube
• Phone switched to the IoT VLAN while Chromecast remains on the 5-port switch -> I can cast YouTube.

Can anyone help me out why my chromecast wouldn't work hard wired when it's connected to the USW 5 port switch?

Happy to provide any further info if it helps