As you know, a ticket is valid for x days from the first flight. So if first flight date changes, the end validity date also changes.

Some airline systems decided to automate this re-validation, but most systems (including Amadeus and Sabre I think) decided that the logic to do this wasn’t worth it, and that with ET, re-issuing rather than re-validating is a simpler approach. But a re-issue triggers checks on new fare, tax & fuel surcharges. Which, again, is not a bad thing from an airline POV.

Once you’ve started the journey, then any changes won’t impact the final validity date, so not a problem