[tptacek]

This only matters if there's an existing protocol that uses truncated SHA-2 you need to interoperate with (and your premise is there is not).

There is no functional advantage to having a different IV for SHA-2 5-2/256.

[baby]

Yup, this looks like it's a cross-domain protection.

[tptacek]

Sure, but if you care about domain separation, you can't rely on the IV anyways.