As in the title, if in the project you have multiple models, you go support only one of them, after finishing supporting you go back to the models, only the supported one is visible, the others disappear, and only the little gray dots that indicate the overhangs are visible. So you need to save the project, close chitubox and reopen chitubox and reimport the project to view the models again.
I haven't been able to reproduce it with precise steps but it's pretty frequent.