This happens with home button when a custom url is set as well, the home button is shown in toolbar, but by clicking on restore in customize page will get it back in the list.
Bug 1695234 Comment 1 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
This happens with home button when a custom url is set as well. The home button is shown in toolbar, but by clicking on restore in customize page will get it back in the list.