I believe I have a similar problem that only seems to occur with AC18 on my Mac with Yosemite. It did not happen before I installed the lastest Mac OS...and it still does not happen on AC 17 with Yosemite but it sure is a drag on 18. I call it an aberration...
The last event occurred when modifying a circle. The screen would shift away from my work and the circle never completed its modification. I tried various times before I realized I was in shift/shape mode. Typically I can reboot the computer and it resolves the problem...for a few hours to a day or two.
I too have tried different settings, different mouses, quitting AC, etc., but nothing seems to work. This aberration problem only happens on 18, NOT 17. And I did not have the problem before switching to Yosemite, I'm current on AC fixes and it still happens. AC 18 is a waste, because I can rarely work a full day without the graphic ghosting double image shift-shape aberrations. And yet I keep trying to use 18...like hitting my head against a wall.
I have no idea what to do...other than stay with 17.
Just 4 Measure, Ltd
iMac Pro 3.2GHz Quad, 32GB, OSX 10.13.6, AC-5, 6, 6.5, 7, stopped for a few years--started again with 12 thru 17, 18 or 19 didn't work, then 20, 21, now skipped to 23. Started in 1976 hand-drafting ink on mylar...