The trustee of a defined benefit plan makes an investment in what seems to be a legitimate investment but is later revealed to be a Ponzi (pyramid) scheme. The plan was an early investor in the scheme and realizes a gain of over $30,000 in the investment.

The investment is revealed to be a Ponzi scheme and later investors bring a class action against to recoup their investments. Can they obtain recovery from the DB plan? Can the DB plan participants sue the trustee for fiduciary breach, even thought the plan made money on the investment?

yes they can, if the investment causes a loss to the plan, e,g. a ct orders the DB plan to disgourge the profits made in the Ponzi scheme. Usually the only the sponsors of a ponzi scheme are sued for losses, not other investors.

What did I miss?

If the Plan makes a profit (as per the original post), What are you referring to when you reply stating "if the investment causes a loss to the plan" then later state "disgourge the profits made in the Ponzi scheme" ?

If the Plan made a profit by being an early investor then there would not be any loss to the Plan. If there is a loss to a Plan then there would be no profits to disgorge.

In the unlikely event that the plan is forced to return the profits, the plan would have earned 0% on the funds. If the investment is deemed to be imprudent under ERISA, the fiduciaries would be liable for amount the investment would have earned if it was invested prudently. In otherwords a loss under ERISA does not require a negative result. A loss includes the amount that could have been earned if the funds were invested prudently. If the plan could have earned $20,000 on a prudent return and instead earns 0 because it had to return the entire profit then the fids are liable for a loss of 20k.