The problem:

When selecting the "Synchronize" button for a diverged branch, when fork syncing is enabled, a window is opened giving the options "merge", "rebase" and "discard". What we notice (Bitbucket Server 5.10.1) is that if and only if selecting "merge" or "rebase" is the warning "Discarding means your changes will be lost" is displayed. When selecting the "discard" option, the option where this message is relevant, this warning disappears.

Merge option selected: https://imagebin.ca/v/4BN7DOhDAVku
Rebase option selected: https://imagebin.ca/v/4BN7JxChVyTA
Discard option selected: https://imagebin.ca/v/4BN7NO8WqBUe