It's not strange at all that your lens could focus well on one body but not the other. Each body and lens has a range of acceptable focus accuracy. Canon repair calls it "within spec", which is ambiguous and annoying. Anyhow, what it does show is that your two 5D3 bodies are not in sync with each other, meaning that the distance of the sensor to lens mount are offset by different amounts on each body. A service tech can add or remove shims of specific thicknesses to compensate for a front or back focusing body. It would be better if both of your bodies were synced in that spec, and it would be IDEAL if both your bodies were synced to ZERO offset, which is considered perfect. The only problem is, Canon repair doesn't calibrate to ZERO, they calibrate to an acceptable range that is "within spec" so it is all but impossible to get them exactly matched. At least, this is my experience with Canon repair, using three different facilities.
You can compensate for this by dialing in specific microadjustments for each body and lens combination. I'm sure this would get your 85L (and all lenses) working very nicely with both bodies.
The only tech I have ever found in the planet who calibrates bodies to zero offset is Toshio. It costs me a freakin fortune to ship my gear over to him, but it's worth it to me to know that it will be fixed right and will work as well as it possibly can.
