my Lo is doing this too, usually on 2 naps she does A times of 3.5 hours then 4 hours then 3 hours but when 1 nap she'll do 5.5 to 6 hours first and then 5 hours after the one nap which is 2-3 hours long, and then the next day or a few days later it's back to 3.5 hours. crazy. i think it just takes her a bit to catch up from just doing the 1 nap. i don't have any other explanation for you
one thing i have noticed though...this transition is taking way longer than the 3 to 2 transition which only took about 3 weeks for us. like i said before she's been doing 1 or 2 naps for about 2 months now. when it started she did a 1 nap day about once a week and now the frequency of the 1 nap has increased and how long it takes her to recover from it has reduced. this week she has done 5 out of 7 of the days 1 nap and just needed 2 days of 2 naps to recover. so she is kind of inching closer to really being just on 1 nap.
i have found it extremely frustrating as i never really know how many naps she is going to do and it has been going on for 2 months now. so i try to guess when she wakes up how many naps it'll be and plan my day but like yesterday i thought she would do 2 naps so i made sure i was home by 10:30 am and tried to put her down but she didn't go so i was home, had to stay home, give her lunch and try again at 12:30 at which point she did finally go down and slept for almost 3 hours. but it totally screwed up all my plans for the day and i was home from 10:30 to 3 which i really would have rather not been.
i don't think they revert. when he's caught up on sleep he'll do another 1 nap day and then you'll have a frustrating period when you never know what he's going to do. don't have any advice for that for you because i'm having a hard time dealing with it myself. try to be flexible i guess and don't get too worked up about it. easier said than done!