Disentangling reaction mechanisms in weakly bound nuclei remains a long-standing challenge, complicated by the common practice of treating absorption as an incoherent sum of channel contributions. Within the Continuum-Discretized Coupled-Channels (CDCC) framework, we apply the generalized optical theorem [S. R. Cotanch, Nucl. Phys. A 842, 48 (2010)] and show that the total absorption cross section, ${\sigma }_{\mathrm{A}}\propto -\langle \mathrm{\Psi }\left|W\right|\mathrm{\Psi }\rangle$, decomposes as ${\sigma }_{\mathrm{A}}={\sigma }_{\mathrm{D}}+{\sigma }_{\mathrm{B}}+{\sigma }_{\mathrm{int}}$, where ${\sigma }_{\mathrm{int}}$ is a coherent interference term between channel components. For the systems and complex fragment-target optical potentials considered, ${\sigma }_{\mathrm{int}}$ is negative and comparable in magnitude to the direct absorption terms. The off-diagonal imaginary couplings play a dual role: they redistribute flux among channels and generate ${\sigma }_{\mathrm{int}}$, which is required for flux-balance consistency. In calculations for $d+{}^{93}\mathrm{Nb}$ and ${}^6\mathrm{Li}+{}^{59}\mathrm{Co}/{}^{208}\mathrm{Pb}$, retaining the full nondiagonal coupling matrix nearly doubles the breakup-channel absorption for the heavy target, while reducing the total absorption through ${\sigma }_{\mathrm{int}}$. We demonstrate that neglecting the off-diagonal imaginary couplings $W_{ij}$ ($i\ne j$) is not merely an approximation but leads to a systematically biased physical picture: the total absorption is overestimated while the breakup absorption component is severely underestimated. Experimental analyses that employ incoherent-sum models to extract direct and breakup cross sections from data will inherit this bias. The full coupling matrix is therefore essential for mechanism-resolved cross-section extraction, and we advocate that experimentalists adopt full-coupling CDCC calculations as the standard for consistent interpretation of absorption data in weakly bound systems.

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• Breakup reactions
• Direct reactions
• Nuclear reactions
• Optical, coupled-channel & distorted wave models