Exchange-coupled hard-soft biphase magnets are technologically relevant systems in that they enable tailoring the magnetization reversal process. Here, exchange-spring behavior is observed in $\mathrm{CoF}{\mathrm{e}}_2{\mathrm{O}}_4$/FeCo bilayers for soft thicknesses as thin as 2 nm, at least four times below the exchange length of the system. This result is in contrast with the accepted theory for spring magnets that states that the exchange length defines the critical thickness below which both magnetic phases should be rigidly coupled. In combination with micromagnetic calculations, this surprising observation is understood as a consequence of the dominance of domain-wall propagation in the soft phase during the reversal process, so far unaccounted for in theoretical descriptions. Our results emphasize the need to expand the existing spring theory from coherent rotation to domain-wall related processes in multidomain configurations in order to accurately design magnetic heterostructures with controllable reversal.