Hi! There is no solution. Support answeared me this:
"Hi, it’s correct, who finished the series does not receive new steps. This is ok, since if I finished the series 12 months ago, it make no sense to receive a new email after 1 year."
I answeared, that for me it is essential that I can add mails to my autoresponder little by little and that ALL subscribers who originally signed up for this list receive it. And I asked to include at least select a “Subscribed before” in addition to “Subscribed after”-option.
Support said that they would check with the dev the option to add that “before” filter. This was the last thing I heared since September the 19th. 😉