= 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 \b[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.
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