Thanks for the logs.
I really feel this is a keying/timing issue, especially when you are using a two-key combination prior to using the numeric key. The following error message appears in the log each time the keying sequence is not activated:-
Altitude change request not initiated due to invalid altitude value!
So when using Shift+\+ 2 PF3 is (some of the time) only recognising the Shift+\ and takes the \ key as an invalid altitude request... so far better to stick to single key combinations preceding the altitude request. As you know, sometimes this same keying sequence worked just fine.
All this requesting lower altitudes though seems a bit excessive to me. You say the MSFS flight planner is causing you problems but there are a couple of ways to address this. Firstly you could use another flight planner. Personally I don't have any issue with said planner and use it all the time. But I do, and this brings me on to my second suggestion, always check my flight plan within PF3
and make any necessary adjustments... like removing waypoints that are (a) too close together (b) are too close to my departure airport (c) too close to my destination and (d) adjusting the descent profile altitudes if necessary, although PF3 makes a pretty good job of doing that automatically for you.