There is some evidence that they have done it becore.

Specifically, that they had a backdoor for Dual EC DRBG[1], and pushed for a shorter key lenghth for DES, to intentionally weaken it.

It is, obviously, controversial. But I think there is certainly enough there that I wouldn't be confident that the NSA wouldn't recommend something they knew to be weak in some way, even to other government organizations.

[1]: https://en.wikipedia.org/wiki/Dual_EC_DRBG