At his BF% (or any skater), it is anatomically impossible to not lose muscular mass when losing weight. Their regimen aims to preserve as much of MM as possible, while dropping them to single digits BF. It's not that he gains bulk, it's that it's easier to see definition. That's what cut is all about and it's a dangerous game when it is sustained for this long, because canibalizing on muscular mass is inevitable. The simplest explanation why he is losing control by mid program is because he dropped too low, went too long too underweight and it impacts his energy level and endurance.
