just corrected my own 'mistake':
I was sending notification e-mails with ticket ID in the subject without brackets :
instead of ID [#8]
(BTW, not essential to have a dash - in front of "ID")
Apparently, the hash # is not enough to identify the ID, so the brackets [ ] must always be in the subject.
If not, if the customer replies to the e-mail (i.e. not having brackets in the e-mail subject either), the system will interpret this as a new ticket, with a new ID, instead of a sibling of the existing ticket & ticket ID.