If I redeem an in-person reward in one calendar year but the event takes place in the following year, which year's Reward Units does it count toward?

Reward Units are counted based on the date the reward is redeemed, not the date the event takes place.
For example, if you redeem match tickets in December 2025 for a match played in January 2026, those tickets will count toward your 2025 Reward Units total, not your 2026 allowance.