This happened to me, and the friend I was playing with, after we both completed Island for the first time; if it helps, I gained two achievements after completing the final hole. (Hope this isn’t a strange format of video, haven’t uploaded video before!)
I gained 3,000 units every few seconds, while my friend gained 6,000 - this continued constantly until we left his condo. Once we entered another condo, I gained 1,500 units every few seconds. My unit count went from about 580,000 to around 900,000 through all of this, stopping (for me, anyway) after I entered a third condo.
Hopefully this is enough information, I can provide any additional details if needed!