"can't have" as in "A mass packed down to neutrons like a neutron star would expand and leave the neutron star phase if it dropped below the mass threshold"? My idea was that once it is packed down to neutrons, it will stay there, and not return to atoms with protons and electrons in the mix. So I thought that this could have happened just like with supernovas that shed an outer layer while the core keeps compressing.

I understand that from the plain mass aspect, you need more tha 1.4 sun masses to compress the nuclei+e soup down to neutrons. But in the end what you need is some energy to do his compression. It might come from mass, it might come from a solar collision, or, what I suggested, that the original mass was sufficiently critical to compress the core down to neutrons, but during this process something happened (radiation energy coming from the collaps of the core?) that blew the not-yet-converted-to-neutron outer parts away. Of course, this idea can only work if a neutron star will stay a neutron star once converted.