This is an extremely strange issue where we found the original solution on Spiceworks.

If you have a disappearing add-in within Excel, the solution is:

If you enable the Add-in while the Analysis ToolPaks were enabled it would disappear on close/reopen.

However, if you enabled the Add-in while the Analysis ToolPaks weren't ticked then went back in to it and re-ticked the Analysis ToolPaks it'd be fine and wouldn't disappear.