deleted sheets are VOID

[carsto]

Initially the workbook had 20+ sheets
I deleted all but 2

I must have done something different when deleting (normally I Right Click & Delete on the sheet tab)

In the Project Explorer window the deleted sheets are still listed and marked void, also the sheet property is Hidden.

I can Unhide them and delete them and they are gone - until I close & reopen the workbookm then they are back and void again!

Does anyone know what I did to mark them void?
And is there a way to unvoid them if they were deleted in error? I can see a benefit in the future to this feature, if I understood it!