Sequence.deleteChars (including gaps) makes incorrect new dataset sequence

Sequence.deleteChars only creates a new dataset sequence if the deletion is 'internal' i.e. doesn't start at the beginning, or end at the end, of the sequence.
However in this case, the same number of characters are deleted from the dataset sequence, as from the aligned sequence.
This is incorrect if the deleted region includes gaps.
Import from Textbox
>seq
AB---CDEFGHIJK
and Cut B---C
The resulting dataset sequence should be ADEFG... but inspection in the debugger shows it is AGHIJ...