I have had many previous experiences with proofs breaking as the toolbox updates. Although not with the v1.6. Also, the obligation you pointed is proved on my machine (it's a 6th gen i5 processor dual core, 8gb ram)
With an ever evolving system like TLA+ I think it is not unreasonable to expect some proofs breaking as versions update. I typically try invoking Z3 explicitly first, especially for arithmetic and algebraic obligations. If that doesn't work, I just break the proof down a bit more and make certain "EXCEPT" statements in the obligation as subgoals and that usually clears things out.
Hope this helps,
Saksham