Usually android kernels have their own cpu regulators and governor settings while mpdecision is for Qualcomm chips and nullifies the kernel's settings, so maybe these two (in Mi4's case) don't exactly get along. Perhaps Miui's MM kernel doesn't "allow" it to function properly, so by removing mpdecision the kernel's settings are "better" for this particular chip.
Or perhaps cpu hotplugging is the culprit and simple tweaks to thermald can solve it.
In any case I guess we will find out if no reboots happen. Maybe it's worth a try just by tweaking thermald and leaving mpdecision too.
Was reading somewhere that Mpdecision is anyway disabled by Xiaomi as they implemented their own CPU management, so removing it would not make any difference.