> For example, with (setq completion-use-base-affixes t)
> if there is some text in the current buffer after point,
> then typing 'M-C-i' and selecting a candidate to insert to the buffer,
> it replaces all the text after point with an empty string.
>
> Before this change, the suffix was set to the text after point,
> and after inserting the selected candidate the suffix was
> re-inserted to the same buffer.

Here is a patch that should support 'completion-in-region':