There's no word yet on why both Superstars were forced to use replicas this week. WWE usually keeps multiple title belts in case of any issues, indicating someone handed the wrong belts to Rousey and Lesnar, or a belt was left at home.
You can see screenshots of both titles below:
I had to rewatch Monday Night Raw and I couldn't help but notice that something was off with Brock Lesnar's belt. Hmm ?? oh it's a replica ?? pic.twitter.com/hzLa6AinJY— Justin (@Jus2Xtreme) January 24, 2019