A quick Google search reveals this problem has been around since launch and has yet to be satisfactorily addressed by the developer of the game, Firaxis (a developer I admire for their excellent XCOM reboot). About the only consistent recommendation for a “solution” is to start a new game, and not move any unit to the edge of the map. For a game that is all about exploration and expansion, this is just too much to take.
I have yet to find any official information on this bug from the developer. I submitted a ticket at 2K Support and will refrain from knocking the game with a bad Steam review (for the moment), but this is shoddy work for a franchise that I had thought was AAA-level gaming. I am beginning to see why it has such poor reviews.
This is a game that I am hoping to livestream at some point, however I don’t care to invest time and energy into something that is unfinished or outright abandoned.
Update 17:25: As mentioned above, I submitted a ticket this morning to 2K and they responded that there is a fix in the works for the intermittent combat sounds that will be part of the next patch, however there is currently no ETA for the release date.
I am quite impressed with a same day response that is 1) on a Sunday, and 2) not a copy and pasted boilerplate rebuttal of my issue.