Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
I am getting inconsistent results running the following in IM 6.9.8.10 Q16 non-hdri. Sometimes the first one works and the second one does not. Other times it is the other way around. Other times both fail or both work. Note for IM 6, the -coalesce seems to be needed. Whereas in IM 7 it needs to be removed. See the recent IM 7 bug report.
Unfortunately, we cannot reproduce the problem under CentOS 7. We ran your command ten times and got consistent and expected results. We'll try under Mac OS X later this week.