My experience last week suggests that's not always the case. However to be clear, my bookings were not dummy bookings - they were trips we intend(ed) to take (just one destination change due to cold feed).

Ticket purchased Fall 2021 US-Italy RT. Cancelled, ticket credit issued, expiration date 12/31/23.

March 2023 - booked new flight US-Peru RT and utilized ecredit; there was a fare difference that was paid with credit card at time of booking.

April 2023 - cancel US-Peru RT, ecredit issued, expiration date 1/31/24 (so 1 month added to original ecredit expiration), and within 24 hours we rebooked US-Portugal RT; small fare difference paid with credit card at time of booking.

This was actually super relevant for us, as our return flight from Peru was on 12/31/23 (last day of validity). When I cancelled, I noticed that it extended it to 1/31/24, which allowed us to book our US-Portugal return flight on 1/1/24 -- which would not have been possible with the original credit.