Whenever I launch Garmin Express 6.3.0.0 on my 27" iMac running OS 10.13.4, my bluetooth mouse and/or keyboard disconnect. This does not happen with any other software on the iMac, including Basecamp.
This bug is consistent and was also happening under 10.13 earlier versions, and an earlier version of GE, but I had not narrowed it down to GE prior to updating to 10.13.4.
-dan