Good question. If you refer to the specification: https://www.bittorrent.org/beps/bep_0027.html
1. In case of multiple trackers entered, the specification demands to drop (and forget) all peers when switching to a different tracker URL.
2. Though there're no hard words against DHT/PEX/LPD aka LSD
At the same time, it says:
it MUST ONLY announce itself to the private tracker, and MUST ONLY initiate connections to peers returned from the private tracker.
This sounds like private torrents are expected to operate in tracker-only mode (without DHT/PEX/LPD aka LSD).
When multiple trackers appear in the announce-list in the metainfo file of a private torrent (see multitracker extension in ), each peer MUST use only one tracker at a time and only switch between trackers when the current tracker fails. When switching between trackers, the peer MUST disconnect from all current peers and connect only to those provided from the new tracker.
And to strictly separate peers per tracker URL. Technically though it could be permitted to talk to a private & public tracker as long as the client NEVER becomes a bridge between a private tracker URL
and a public tracker URL.
Tixati should not have PEX enabled and hopefully doesn't have PEX enabled for private torrents. The checkbox should've been inactive too, you are right.