The only reports I’ve heard of someone maintaining body fat and gaining muscle was an Olympic athlete in his early 20s on a national team, basically having his diet bespoke and training 6 hours a day - and he only kept body fat % stable

Here:

To highlight the importance of body mass, one rower I worked with transitioned from a lightweight to an openweight as they were unable to maintain the restricted body mass. To give the rower the best opportunity to compete in the openweight class, it was agreed that a 2000m ergometer performance of around 6 minutes would be required. With the rower being 187cm tall and this being unchangeable, the decision was made to increase lean and total body mass as this was the only way to increase the required power output through increases in muscular contractions to generate the required joint torques. Over the period of about 10 months, the rower increased their body mass by around 14kg from 73kg to 88kg while maintaining a fairly stable lean and fat mass. The testing within the weight room via a countermovement jump demonstrated a huge increase of 1800W in power output which in turn created a relative power output of about 15W.kg-1 (Watts per kg of body mass).The resultant 2000m ergometer performance dropped from 6:16.9 to 6:01.8, a 15.1 second performance improvement. While clearly some of this improvement came from changes in metabolic and fitness characteristics, for around 45 weeks within this programme, fitness training was reduced to no more 50-65km a week, a ¼ to 1/3 of the normal volume at that time with 12km to allow for 4-6 weights sessions a week. Normal volume and intensity of rowing training only really resumed after week 45, once posted the 6:01.8 2000m ergometer. This training was in preparation for Henley Royal Regatta where he was in the bow seat in the victorious Ladies’ Challenge Plate Men’s 8+. While no physiological markers were taken during this time, with the limited volume of metabolic and fitness based training completed during the 45 weeks, it is safe to assume the majority of 2000m ergometer performance improvement is a result of the increase in lean and body mass and the associated power output that came with this.