Trustees of an ERISA health plan sued the owners of Accuracy Glass & Mirror Co who had not paid contributions that were required by two master labor agreements. The trustees claimed that the unpaid contributions were plan assets and that the owners had breached their fiduciary duties.
Funny thing though. There was a previous bankruptcy case that held that there's no fiduciary duty until the employer pays its contribution over to the plan. (That case was Bos v. Board of Trustees (Bos I), 795 F.3d 1006 (9th Cir 2015) [PDF].) The 9th Circuit held (2-1) that it was bound by that case, and thus held that the owners were not fiduciaries as to the unpaid contributions. "[E]ven an ERISA plan that treats unpaid contributions as plan assets does not make an employer a fiduciary with respect to those owed funds." Glazing Health and Welfare Fund v. Lamek (9th Cir 03/21/2018) [PDF].
A dissent took the majority to task for extending a bankruptcy case out of the bankruptcy context. "I would find that unpaid employer contributions to employee benefit plans may constitute plan assets when the ERISA plan document expressly defines them as such."
There is a split of authority among the federal Circuit Courts on this issue.