GPU disabled only on one file, other file works normally

Previously I have one file that works normally with GPU preview. But after playing around with Blend Tool and Pattern Brush (repeating simple vector rectangle, but far from crazy amount of repetitions; under 25 object repetitions). After that the GPU preview gone, and replaced by CPU preview. Scrubby zoom is disabled as well. 

BUT other file works just fine. I open different files, the GPU works normally.  

I had Reset preferences, and after restart, the problem still exists. Only that file has the problem. File damage perhaps?

I did Save a copy, but the problem persist. The only workaround right now is simply by copy-paste the graphics into different file.

I just wondered what’s the problem here? Why only one file has the problem?