I recently installed a new Unifi Dream Machine, placed many of my IoT devices onto a separate VLAN. A few items, such as Hue and HomeKit devices still work. However, all my TP Link devices on HA have stopped (but they still work in KASA).
Is there a setting I may be missing to allow HA to discover my devices on my IoT subnet?
Try manually configuring them instead of relying on discovery.
That would require manually static coding IP addresses for every switch and plug (about a dozen). The point of discovery and DHCP is not have to worry about that.
Kasa sees them across subnets. Why can’t HA?
True, but it’s a “set it and forget it” type of thing. As far as why the kasa app sees them but not home assistant, do those devices still have internet access? If so, the app might be seeing them because of the cloud connection.
Cloud connection, whereas HA is local. You can set HA to use a broadcast address, but I’m unsure if it supports directed-broadcast (and even if it did, your home network would need to support it too).