Do significant strikes landed after the horn count in a UFC player-stat prop?

Short answer

An attack after the round-ending bell is a foul under UFC rules, not an extra legal exchange in that round. For a significant-strikes wager, use the named statistical provider’s final total and the operator’s correction rules. Do not add a visible after-horn strike yourself or assume every contact shown on television is a credited significant strike.

UFC Unified Rules: attacking after the bell