This sounds completely plausible! Apparently, that mount reports that it would flip if slewed past that limit. That's what ACP is seeing and interpreting. By preventing ACP from triggering the mount's "last chance" slewing limit, you'll avoid triggering this quirk, and very likely be home free. ACP will error out on trying to continue beyond the horizon limit. There are others here who use this as a "poor man's" scheduler :-)