Multi--stability in the response of a ferrimagnetic spin resonator to an externally applied driving is experimentally studied. The observed multi--stability cannot be derived from any master equation that linearly depends on the spins' reduced density operator. Traditionally, the nonlinearity that is required in order to theoretically account for the observed multi--stability is introduced by implementing the method of Bosonization. Here, an alternative explanation, which is based on the hypothesis that disentanglement spontaneously occurs in quantum systems is explored. According to this hypothesis, time evolution is governed by a master equation having an added nonlinear term, which deterministically generates disentanglement. Experimental results are compared with predictions derived from both competing theoretical models. It is found that better agreement with data is obtained from the disentanglement--based model. This finding, together with a difficulty to justify the Bosonization--based model, indirectly support the spontaneous disentanglement hypothesis.