I am using 3.65.03. I use autocomplete regularly so disabling it is not an option. When I 'delete' the unwanted part of the string by using the backspace key, the unwanted part just pops back into place - this happens no matter how many times I delete it.
I even try to delete the unwanted part and then immediately press tab to place the cursor in the body of the e-mail but the autocomplete still fills in the subject with the unwnated bit of text.

It seems once you enter a string you can never shorten it - the full original string keeps adding itself. Deleting part of the string should be final.