In my opinion, it seems unnecessary to complicate the discussion with a scenario that you admit you've never seen. Even so, I'm sure that we agree the investment gains or losses do not change the amount of the plan contribution, which was the point of the first three responses.
Does it not seem much more likely that we are talking about a pooled investment account scenario based on the fact that the "trustee decides to do an interim valuation"? In that case, the issue of unallocated trust subaccount versus allocation participant account seems irrelevant.