Just write a polite (private) email to the PM(s) in question explaining the issue like you did here. IMO the tone of your post is neutral and nonoffensive, it is not blaming anyone personally, just pointing out the problems caused by the specific behaviour, and the way to avoid them. The only exception to me is the word "hijack", which I would replace with a more neutral expression.
A general message template for nonviolent communication could look like: "When you do ..., it causes problems to ... due to ... . You could avoid negative effects by doing ... instead."