I recently made a holder for a part using the Insert Part and Combine features, but when I mirrored the mounting tab, the inside bottom became flat. I can't figure out why it did that, although the holder will work flat for me anyway. I'm just wondering what I did wrong so I can do it right in the future.

Here is an iso-type section view of the holder with the Mirror feature rolled back:

Here is a section view straight on. Notice the curved bottom:

Notice the flat bottom after I turn on the Mirror feature, it seems to be on the same plane as the top face of the mounting tab:

Here is the feature I mirrored:

Here is the whole part for reference