Teams Premium and Copilot for M365 transcribing issue

Having a weird problem. A reseller of mine has M365 E3 and Teams Premium and Copilot for M365. If he calls someone 1 to 1 he can both transcribe and use Copilot. If there is one more person in the Teams call, it won't work. Same if he sets up a scheduled meeting. What could be causing this? Are there any known steps to troubleshoot this scenario? A fresh install on a different PC with everything in English (no native language) didn't change the issue. They didn't have this issue prior to Copilot deployment, with just Teams Premium.