Ah, that's weird and shouldn't be possible unless there's a glitch/safety override you found in the software accidentally. Anyone else ever heard of such a thing? Anyway to look that up in the tune?
I think what's happening is, even with OD OFF..and in 'drive', the 5r55s + a 3.31 rear gear, and 27" tall rear tires, the car will do 149.9 mph @ 6 krpm. OEM rev limiter is set to 6250 rpm. I got my tune from VMP. He re-tweaked the rev limiter to 6500 rpm. Shift points (gas mashed) were aprx 6 krpm. He unlocked the SCT-X3 such that I can tweak each shift point up /down on a shift by shift basis, like 1-2, 2-3, 3-4, 4-5. The car also has a speed limiter built into the oem FORD software tune, and it's set at 150 mph. What's supposed to happen is the eng is supposed to go into limp mode, with every 2nd cylinder's injector shut off, and ditto with every 2nd cylinder's spark plug.
VMP disabled the 150 mph max speed limit, so there is no max speed limit on my 2010 auto. One would think with the OD OFF, it would just hit the rev limiter, but apparently the OP sez his will shift into OD by itself. I have only had the car up to 137 mph, so can't test that theory.
JJR427 runs his 2010 auto (with roush M90 blower) on road courses. When the 5r5ss gets too hot, it will shift into OD (even with it off) at 70-120 mph. In OD, the rpm drops of course, and the idea is the tranny will be less likely to overheat.... it's trying to save itself.
In my case, I didn't like the rev limiter set to 6500 rpm, so reduced it in the SCT-X3. The next avaialble increment down from 6500 rpm is.... 6250 rpm, so set it for 6250 rpm. Fine, but noticed..twice now, when maxed out in 3rd gear, it would hit the rev limiter ! (instead of shifting to 4th). My 'fix' for that was to lower the shift point slightly for the 3-4 shift.... problem solved.
In JJR427's case, he had to install a 2nd tranny cooler. I also installed a 2nd tranny cooler, ( in series with the oem tranny cooler). These are the B+M type, 3/4" thick, and are bar + plate config...(instead of tube + fin). New cooler is installed right below the oem cooler. I had also installed a PA deeper pan, which is 10.5 lbs..and is exactly 2.1" deeper than oem steel pan. And also the JDM engineering catch can for the 5R55S. And also RP 'max atf' 100% synthetic ATF. With the deeper pan holding an additional 4 qts, I would have thought that would have solved my high temps on the 5R55S, it didn't.
With blower on, on the hwy, auto tranny temps (as indicated on the aeroforce digital gauges plugged into obd port, and gauges mounted on the SOS A pillar pod), the tranny temps would go from 170 F..to 200 F in a mere 5-8 seconds. After the 9700 BTU B+M cooler installed in series with the oem cooler, the temps only rise to 171..and sometimes 172 F.
But JJR427 still had a 5r55s overheating problem when on a road course, like during the 3rd session of the day. And he installed the even bigger 20,000 btu B+M cooler in series with the oem cooler. He had to cut out a chunk of the front bumper to get it to fit. With temps > 200F, the 5r55s will shift into OD ( even with OD off). I don't road course, so my 9700 BTU extra cooler, catch can, and deeper pan + synthetic is fine for my street use.
The wind resistance > 100 mph is wicked. And wind resistance goes up to the square of the velocity. 200 mph is quadruple that of 100 mph. So you can see why the car bogs down when > 125 mph. It's maxed out at 150-160 mph in 4th gear. In OD, the eng rpm drops like a rock, and not enough power left at the lower rpms, to overcome the wind resistance. It would never make it to 170-200 mph in OD.