Ironically i think that just happens sometimes when you have really good boost control. Here's some examples from my logs:
https://datazap.me/u/thejeremyman9/1...&data=4-5-7-17
https://datazap.me/u/thejeremyman9/d...=4-5-7-9-17-19
I think its happened less since i went down to XHP stage 2 but i havent logged 4-5 gear shift in a while.