Merge of rebase causes commits to be overwritten

We had an old branch, feature-x, and we tried merging in a recent master. In the merge we saw t开发者_StackOverflow中文版hat a bunch of the commits from the feature-x branch had been overwritten, and the code had disappeared. I'm now cherry-picking the relevant commits into a fresh branch off master, but I was wondering why this happens. Anecdotally this also happened to me in SVK... is there something we're doing wrong?

That should mean those commits were already merged at some point in master.
Meaning any future merge won't pick them. And cherry-pick is then a way to force them back (again) in master.

A radical solution would involve a merge with an 'our' merge stategy, albeit in reverse. (but that would then override all files from master with those of feature-x)