Definitely the case that Travel Credits are forfeited if you cancel a booking.
In fact, it seems a mission to get a Travel Credit returned even if your booking is cancelled by the airline.
I had a Jetstar flight from Sydney to Honolulu a few months ago that Jetstar cancelled the night before due to issues with their 787s. I'd paid using a Travel Credit and cash for the excess via AMEX. Jetstar immediately offered a full refund and even sent me confirmation it had been processed, but this never appeared back on my AMEX or as a Travel Credit.
When I followed up with AMEX, they insisted it would take 8-12 weeks to return the cash (even though I had confirmation the refund had been processed back to AMEX by Jetstar) and that the Travel Credit was forfeited even though the cancellation was involuntary and from the airline's side.
After speaking to several AMEX reps and questioning this, I was finally told the Travel Credit would be returned "as a courtesy" as if I'm to be very grateful for their generosity. It has not been, even though the cash portion of the ticket did finally go back on my card two weeks ago. I've followed up several times again and each time the entire experience is like speaking to robots. I'm told to wait "8-12 weeks for the refund to be processed" and each time it's stressed that it's a "courtesy" I'm getting the Travel Credit back at all. It's as if speaking to a wall when I point out Jetstar has already refunded the value and/or ask how it's possible that the cash refund has come back but the Travel Credit hasn't. Clearly the cash coming back indicates the refund has been confirmed and processed even on AMEX's end.
It doesn't matter whether I take a very self-deprecating, kill-them-with-kindness or a more blunt, no-nonsense approach, I get nowhere. I generally like to imagine I manage to maintain my composure and patience with these kinds of things, especially after Covid, but this one has tested me --- perhaps not least given the high cost of the card.
I am closely reassessing whether to renew my Platinum in February.