There is a trimming routine that removes or changes certain punctuation at the end of a title. For example if you try to end a title with
... it will be removed from the title, when you hit submit. This is not the subject of this question.
It seems that when you edit a question the trimming routine is performed on the title regardless of whether changes were made to the title.
This means that when you make changes to the body of a question it can also trigger unexpected changes to the title. For example when you edit a question, whose title ends with any number of consecutive periods (e.g. 3 in the case of
...), the periods will be removed from the title, even when no change was made to the title itself.
In order to leave the title unchanged when editing you must add an extra
. or something, which itself will be removed. You have to know to do this ahead of time, because the UI still displays the original title until the edit is submitted.
It's rather difficult to notice this is happening unless the trimming brings the title down under 15 characters in which case you will be prevented from submitting your edit with the message "Title must be at least 15 characters".
This can be very confusing. You haven't made any changes to the title, but the UI won't let you submit your edit.
I do not believe this is the intended behavior. I think either trimming is expected to be idempotent, in which case this issue would be impossible, or trimming should not occur when no change is made to the title.