I can't get this to reproduce and its confusing to me how its possible. The trace in T7795 uses the "LOAD" pathway on the update controller. Under the hood, this issues a ThreadQuery with needTransactions to true. With needTransactions to true, the transactions and pertinent handles are all loaded nicely.
So... best guess is there has been some LIMIT of transactions since the offending person participated...? Alternative fix which would probably work is to specify needParticipantCache to true.
More on T7795 - the user report found the "a, b, c..." subtitle thing in the messages dropdown confusing. Yet another fix here would be to change that to be something like "a: snippet of what a said...". I'll discuss that on the task.