I tried Mighty777's 1.5.6. 50Mbps first and found a file split at 7-9 MB and the remainder of the filming with a very high kbps rate for the first and 35-36 for the remainder . I think a slow card could have caused the split.
I don't think is MicroSD card the issue of splitted file and reduce of bitrate. I have Samsung 64Gb Evo Plus in it, that can reach over 60Mb/s Write speed . I rather think is something about the Xiaomi temperature, when it write that much data it gets quite hot... actually very hot.