A search for new resonances decaying into a pair of bottom quarks in the merged regime is reported using an integrated luminosity of 80.5~\ifb of proton-proton collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the Large Hadron Collider. For the resonance to be merged, an extra jet with high transverse momentum is required. The mass range from 70 to 230 GeV is inspected. For the Standard Model Higgs boson, the observed signal strength is $\mu_{H} = 5.8 \pm 3.1~\text{(stat.)}~\pm1.9~\text{(syst.)}~\pm 1.7~\text{(th.)}$, consistent with the background-only hypothesis at $1.6$ standard deviations. No evidence of a significant excess of events beyond the expected background is found and limits on leptophobic $Z^\prime$ bosons with democratic axial couplings to all quark generations are set in the probed mass range.