Rim War has a built-in integration with Empire, making the empire settlements a "vassal".

This vassal generates points that are used when defending against Rim War attacks (Scounts, Warbands and Flying Warbands). Empire attacks still happen independently if not disabled.

Edit: When using rimwar and being attacked by rimwar assets (scouts, war parties and drop pod war parties) you don't get a notification since it doesn't use empire's mechanics, it uses rimwar's own.

We will not fix this, sorry for the inconvinience but please stop reporting this as an issue.
It's something you will have to accept happens if you decide to use both mods.

Important: I personally don't recommend playing Rim War with Empire, vassals generate points slowly, meaning attacks from Rim War are more likely to succeed and cause less profit, happiness and loyalty from your settlements.

The decision is yours, but take into account that using both mods means settlements will be a lot harder to manage.