All decent shock pumps are zero loss- there’s actually pretty few on the market that aren’t. People often get confused though because even with a zero loss pump you generally get a hiss of air as you remove the pump- that’s not escaping from the fork, it’s escaping from the pump! The air in the shock is under the same pressure as the air in the shock.
Then, they reconnect the shock and discover the pressure’s gone down. But that’s because when you reconnect, the shock gets repressurised from the air spring, and so the pressure in the shock drops.
Rockshox and Beto one are zero loss for sure. Haven’t used a Fox one though.