I've recently been migrating Tvheadend documentation over to a new Docs/Wiki site https://docs.tvheadend.org and one of the text blocks I've moved talks about how multi-tuner setups on Raspberry Pi hardware can be troublesome. The issues flagged are power; as often you need to run tuners from a powered hub to ensure the current draw is handled instead of being directly connected to the Pi board, and bandwidth as this arrangement results in the tuners being connected to a single USB port. Then for extra credit it mentions that RPi 0/1/2/3/4 boards are sharing USB with Ethernet. RPi5 runs Ethernet from a proper PCI bus which should eliminate the latter issue, but the other issues are still possible.
What happens with one tuner connected directly and other tuners connected via the hub? (experiment with different combinations)