The exotic $\beta$-delayed proton emission is calculated in ${}^{11}\mathrm{Be}$ from first principles using chiral two- and three-nucleon forces. To investigate the unexpectedly large branching ratio measured in Ayyad et al. [Phys. Rev. Lett. 123, 082501 (2019)], we calculate the proposed $(1/2^{+},1/2)$ proton resonance in ${}^{11}\mathrm{B}$ using the no-core shell model with continuum. This calculation helps to address whether this enhancement is caused by unknown dark decay modes or an unobserved proton resonance. We report a branching ratio of $b_p=(1.3\pm 0.5)\times {10}^{-6}$, suggesting that its unexpectedly large value is caused by an unobserved proton resonance in ${}^{11}\mathrm{B}$.