  We report on measurements of the dynamics of the collective spin length
(total magnetization) and spin populations in an almost unit filled lattice
system comprising about 10^4 spin S=3 chromium atoms, under the effect of
dipolar interactions. The observed spin population dynamics is unaffected by
the use of a spin echo, and fully consistent with numerical simulations of the
S=3 XXZ spin model. On the contrary, the observed spin length decays slower
than in simulations, and surprisingly reaches a small but nonzero asymptotic
value within the longest timescale. Our findings show that spin coherences are
sensitive probes to systematic effects affecting quantum many-body behavior
that cannot be diagnosed by merely measuring spin populations.
