Tyson has perfect posture.
Though he regularly falls back on parade rest from his army days, Tyson learnt to hold himself correctly from a much younger age.
Posture was a large part about having to present himself as a proper heir to the Delaney family legacy, and is the leading cause for Jethro regularly hunching over as an ingrained defiance; a bad habit that James ends up mimicking.
Those lessons were further reinforced by dance - a mixture of ballroom, ballet, classical, tap, and contemporary - and fencing lessons that required Tyson to keep good posture and positioning. Various other sports also played a lesser role in reinforcing that.
Following on from Tyson’s struggles to release his control enough to properly balance, Tyson struggles with slumping. Kinda. He can curl and nestle back into anything regardless of the angle it puts him in, Tyson will pretzel himself without even noticing, but he can’t comfortably hunch over without something to lean against/into.
It feels unnatural and stiff, despite his usual rigid posture.