 Solution

A plausible ionic catalytic cycle is reversible thiocyanate addition to the activated maleic-acid double bond, with proton transfers, followed by rotation about the resulting C-C single bond and elimination that regenerates thiocyanate and forms the less crowded trans alkene.

The retained vinylic labels pass from approximately sp${}^2$ toward sp${}^3$ at the addition stage. Stiffer bending modes in that direction give an inverse secondary kinetic isotope effect, consistent with $0.86$. The cycle changes alkene geometry without requiring cleavage of either original labeled C-H bond. Since two positions are labeled, $0.86$ is a combined effect; equal independent contributions would each be $\sqrt{0.86}=0.927$.

The isotope effect supports an addition/elimination route rather than simple rotation of an intact double bond; it does not, alone, identify every proton-transfer order.