Edit: it is veery strange to see a pension fund buying bitcoin directly. Probably theree is some specific legislation in their state to allow such thing, which is very weird and not common, even in us.
It is an exception
I haven't researched if that's the exception, but I don't think there might even be any particular rule that would prohibit pension funds from investing in Bitcoin or anything else when it comes to the US. We also have an example from Canada where a large pension fund invested $95 million in FTX, which only tells us that such funds have great freedom when it comes to the assets of their clients.
The fund said it invested $75 million in FTX International and its U.S. entity FTX.US in October 2021, and then another $20 million in FTX.US in January of this year, according to a statement.