Thank you for reporting a bug to Unity.
We have reviewed the issue carefully, and in this case, the team is unable to prioritize fixing this bug. We changed the approach in TextCore to stay away from a Material approach, which enabled multi-threading for us. This feature was not ported correctly, and fixing it at this this involves a lot of risk and work. However, in 6.3+, you can start using ATG (Advanced Text), which fully supports this feature, and more!
If you are blocked on 6.0, we suggest adding a proper bold fallback to your font, which will have bold correctly rendered on it.