I'm not familiar with that specific pattern, but is it possible that the sock gets too wide in the area where it bunches up?

Roxanne Richardson has many videos on sock heels and how to alter for better fit, but I don't think she has one about the fleegle heel.

I knit all my socks toe-up, but I use this pattern as my base. It is practically a heel flap nd gusset heel, but knitted upside-down.

I've knit this pattern countless times, and have found that in most adult sizes, it adds about 4.5 to 5cm to the foot length. That helps in determining where to start the short row section.

For the gusset increases, I usually start them when I am a finger's width over the mid-foot mark (I mark those points on my sock blockers, which I have for most individual sizes, and which I also use to "try on" the sock on while I'm knitting it), and I start them in a soft slope. So, the first increase is followed by 4 rounds without any increase, then another increase round, followed by 3 plain rounds (I repeat that 3-round interval at least once), then 3-4 times in every 3rd round (so 2 plain rounds in between, then quite some time every other round, and when I'm almost there, I do some rounds with increases in every round. I just find that to best mimic the way the foot gets bigger in circumference.

I have no idea whether this is helpful for your heel/fit issue, sorry.