If the backdoor was obvious then it wouldn't be a backdoor. It would be a front door.
People found out about Dual_EC_DRBG due to the weird funding associated with it.
I look at trends in these communities, and when people who normally disagree on something suddenly shift to it in lockstep (like we saw with e.g. Arch Linux moving to systemd at the same time as all the other distros) I just assume it's compromised.
Here are the RISC-V members: https://riscv.org/members/
It started as a tiny project to get away from the big players' bloated hardware (and their security problems), yet look how the big players have latched on. Proof? No. But definitely something that should make you wonder.