The Hall effect under in-plane magnetic field has emerged as a sensitive probe of quantum geometric properties in solids. In trigonal systems, the in-plane anomalous Hall effect (AHE) originates from unconventional off-diagonal coupling between the magnetic field and the Hall vector on the principal plane. Here, we investigate this coupling in trigonal antiferromagnet ${\mathrm{EuCd}}_2{\mathrm{Sb}}_2$ thin films by systematically tracking the (001) plane across distinct magnetic phases. Around zero field, anomalous Hall resistivity as well as out-of-plane Weyl points splitting shows pronounced magnetocubic dependence, persisting not only in the paramagnetic phase but also in the antiferromagnetic phase. Above the magnetic ordering temperature, the off-diagonal octupolar tensor component decays anomalously with an approximate inverse-cubic dependence on temperature. In contrast, in the forced ferromagnetic phase, AHE is governed by dominantly magnetolinear dependence, continuing to increase up to very high fields. These findings establish multipolar nature of off-diagonal couplings underlying in-plane AHE, broadening the scope of Hall physics beyond conventional paradigms.

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• Hall effect
• Thin films
• Magnetotransport
• Molecular beam epitaxy