Not recognising default Thunderbird signature separator

I’ve searched the support form for this in several ways–I apologise if it is there and I can’t see it.

The default Thunderbird signature separator is “– ” (i.e. –<space>). I have this in my separator divider section in postie, on its own line, and I’ve double checked that the space is there. But no matter what I try, I can’t get postie to recognise the “– ” in Thunderbird messages. I can get it to recognise other things (like “-dan”). any ideas?

I also checked whether it would recognise the sequence “– ” anywhere in the message, and it doesn’t seem to.

https://www.ads-software.com/extend/plugins/postie/