Černý bound for difference DFAs in Euclidean space

Let $M=(Q,\Sigma,\delta)$ be a difference DFA in $\mathbb{R}^d$ on $n$ vertices, with $n$ states and a universally reachable state. A synchronizing word is a word that maps all states to one state.

Difference-DFA Černý conjecture. The DFA $M$ has a synchronizing word of length at most

$$(n-1)^2.$$

This asks whether the Černý bound holds for the stated class of difference DFAs without the additional assumptions discussed earlier in the paper. The source gives no resolution, so this remains open.