  The presence of both inversion ($P$) and time-reversal ($T$) symmetries in
solids leads to a well-known double degeneracy of the electronic bands (Kramers
degeneracy). When the degeneracy is lifted, spin textures can be directly
observed in momentum space, as in topological insulators or in strong Rashba
materials. The existence of spin textures with Kramers degeneracy, however, is
very difficult to observe directly. Here, we use quantum interference
measurements combined with first-principle band structure calculations to
provide evidence for the existence of hidden entanglement between spin and
momentum in the antiperovskite-type 3D Dirac material Sr$_3$SnO. We find robust
weak antilocalization (WAL) independent of the position of $E_\mathrm{F}$. The
observed WAL signal at low doping is fitted using a single interference
channel, which implies that the different Dirac valleys are mixed by disorder.
Notably, this mixing does not suppress WAL, suggesting contrasting interference
physics compared to graphene. We identify scattering among axially
spin-momentum locked states as a key process that leads to a spin-orbital
entanglement, giving rise to robust WAL. Our work sheds light on the subtle
role of spin and pseudospin, when both could contribute to the same quantum
effect.
