How to reproduce:

1. Place command block
2. Set command to something short like "/say hi"
3. Set command to something longer than 256 characters and hit 'Done'
4. Open command block again, see how command has reverted to "/say hi"

The problem here is that players are able to enter text past the 256 char limit, this is very confusing. I suggest either changing the field to not allow more than 256 characters or increasing the maximum command length again and then changing the field to not allow more characters than that. Now with /summon and the new features of /give, 256 characters can be used up very quickly and hinder /give and /summon from living up to their full potential.