I am happy to clarify the state of affairs with respect to the issue being discussed on this thread. I'm sorry it has been a source of pain. It is a priority, and fixes are being developed. To sum things up, the problem is:
After certain events, a non-MS390 switch stack can enter a state where it is unable to route traffic to a particular next-hop (both intermediate and final).
With that in mind, the next question: Which "certain events" can produce this condition? Two known root-causes exist:
(1) When the MAC address of a given next-hop ages out of the CAM table, there is a chance (but not a guarantee) that routing to this next-hop will fail.
(2) When the MAC address of a given next-hop is flushed from the CAM table as a result of a port cycle or STP Topology Change, the same chance that routing to this next-hop will fail exists.
Regarding the status of a fix for these issues:
The first root-cause (CAM age-out) is fixed as of MS 11.29. If your network is relatively stable (free from cycling on non-access ports and STP TC events), then you'll see a big improvement by upgrading.
A fix is being developed for the second root-cause (STP TCs and port flapping). Currently, there is a closed beta firmware release with this fix. If you reach out to the Support team, they can apply this firmware to your network.
Additionally, it is possible to work around the second root-cause without the final firmware fix, if the need arises. Most switch stacks with L3 enabled likely do not experience frequent port flapping due to their position in the topology. So the main way to encounter the problem is when the stack receives inbound STP TCs. To work around that possibility, you can disable STP on the ports which lead to other switches or devices that participate in STP. A port which has STP disabled will not process an inbound TC received on that port.
That's of course not without some risk, as if a loop is introduced to the network, it won't be contained by STP. But if you know your network topology is stable, it is an option on the table.