If you have several founders and stocks are allocated, is it ethical or even legal for the majority stock holder to simply absorb the stock of the ones that dropped out instead of redistributing it proportionally to all of the remaining stockholders?
Short answer: probably not, but your situation may be different.
With almost all vesting agreements, when a founder leaves, their shares go back to the company treasury. They are not redistributed proportionally, but the net result is the same as if they were redistributed proportionally, because now they are taken out of circulation, so everybody who is left owns a larger portion of the outstanding shares.
You should study your incorporation documents, bylaws, and shareholders' agreements to see what provisions (if any) were involved, because your company may have different rules.