Too many hypothesis here.
Saves are not always affected by updates, whether it be Early Access or not.
Sometimes they can.
The probability during Early Access is higher that a past save can no longer work, but we cannot predict or know in advance.
We can make educated guesses based on the two RPGs Larian has already taken through Early Access. They deliberately break saves between major updates in early access to ensure that bugs from version A which get patched in Version B don't get incorrectly re-reported from people using save files created under version A in version B. That's one reason.