While I was updating info for the season, the premiere year which was already added got removed on its own and I couldn't add it back as the year 2020 is not in the list.
This was done because it was decided it's redundant information.
If it's in the past then you can add the episode and then use episode 1's airdate for premiere
Indeed, that property is only meant to be used for future dates. Quoting the policy:
For seasons that have already premiered, their premiere date is automatically calculated based on the first episode's airdate.
So I understand the behavior might be a bit confusing, but we don't consider it a bug :)