I had this happen when I opened AC9 projects into AC10. It drove me nuts for a couple of days before I figured it out. I'm now having a heck of a time remembering what I did. I think I went into the Options/Element Attributes/Zone Categories menu and reselected a zone type in that window. If that doesn't work I'll try to remember what else I did.
Mac OS12.6 AC26 USA Silicon
M1 Macbook Pro