You do not automatically forfeit all rewards by unstaking before the completion of a 90-day Cycle.
If you are Staking and wish to withdraw your staked ROOT in the next cycle, you can begin the 28-day unbonding period at any time before the end of the cycle and continue to earn Staking rewards until you withdraw your ROOT, providing you wait until the start of the next cycle.
If you are Nominating, you will maintain all rewards earned to that point and continue to earn staking rewards, but once you start the unbonding process, your reward rates will decrease to the Staking reward rates until you withdraw your ROOT. As mentioned above, providing you wait until the start of the next cycle to withdraw, you will not forfeit rewards.
If you withdraw your unstaked ROOT before the completion of a full Cycle, you will forfeit all rewards except Vortex rewards earned from Work Points. For more on how Work Points contribute to Vortex rewards, see the infographic in The Root Network Docs Hub.