Hi MIchael,
the automatic reset of the starting date for a second time stamp on works
if the command `org-time-stamp' is invoked twice in direct succession. So if you press `C-c .' to insert a time stamp, and then you press it immediately again, the magic will take its course. This is probably the situation that you remember.
However, if you use %T in a template, then `last-command' is `org-capture', and the time stamp command does not know that you are making a range. I could, by looking back and finding <...>-- , but that is not implemented. You might be better off Using an empty template and then inserting both time stamps with `C-c .' - which will make it easy to also directly choose the starting date.
Carsten