If I had to guess I'd say that there are 2 root causes of this:

1) The cam cover in question isn't flat and

2) There is really an insufficient number of hold-down fasteners. There are the same number (4 for those not in the know)) as the 616 has and that uses a 20% shorter cam cover. I suspect that the 4 on a 616 are marginal and on the 617 are really not enough. When I build my back-up engine I'll puzzle out a way to add more fasteners and see if that helps (maybe it won't matter but I can't know 'till I try it).

Dan