Don’t pair the trainer to the phone via the Bluetooth settings. If it is paired there, unpair it. Zwift does its own internal pairing.
I would also counsel to ensure the trainer is “forgotten” everywhere else - all other apps and devices.
FWIW, my friend has a Tacx Flow Smart and I have a Pixel 5. Next time I’m over there, I’ll try to replicate the issue.
For more connectivity troubleshooting ideas, see this thread: