The whole problem with this behavior is that it doesn't do anything to solve the problem or improve the situation. It just keeps everything in a state of suspended animation, with no resolution. This behavior can put an organization into gridlock, slowing performance to a crawl, suffocating innovation and creativity.
What is the solution to withdrawal and avoidance? First, recognition that there can be no progress without resolution of the issue. Putting off the resolution only delays the time until improvement can occur. Most people don't want to hurt improvement and progress. Often just the recognition that the organization (and your own peace-of-mind) will be better served by action rather than by inaction is enough to allow you to overcome your desire to crawl into a hole and allow you to face up to the conflict.
Second, you must actually discuss the undesirable behavior with your protagonist. After escalation or invalidation a short emotional cooling-off period is often required, but don't wait too long to get the subject on the table, discussed and dealt with. If you find it hard to start the conversation, it can be helpful to acknowledge your feelings, e.g. "This is hard for me to say, but I think it is important that we discuss this. I am uncomfortable leaving it buried. I think we should talk about it for the benefit of the organization."
You will be amazed at how often the other party follows such an introduction with an apology of their own. This clears the air and lets you get back to work on making things better.