This is a common problem and a known issue that’s been happening for a while now. A simple solution would be recreating the union that’s been corrupted regarding that’s a only way. In your case I wouldn’t use a union for something that simple, making a circle around a cylinder could be produced with using a plugin such as ’ Archimedes Two’.
Try to minimize on using unions for simple objects; you’ll have to remake it, this is a common question that’s been discussed in previously threads. Here is a tutorial that goes in depth about recovering lost - corrupted unions.