I may have jumped the gun with my original post.
I cancelled a second booking today, and the resulting e-credit's expiration is per the old rules: one year from original date of booking.

The ticket that I cancelled where I got a five-year e-credit may have been one where there had been a large schedule change. I say may, because I noticed that I have several tickets that have had large schedule changes, but until now, in a world without change fees, there wasn't a huge difference between changing/canceling a ticket with a large change vs. one without one, so I wasn't tracking which particular tickets have had such changes.

The distinction about whether a large schedule change has happened is apparently important again.