The answer I thought I would get, but well now I know that YOU know. It is certainly not a game breaking issue, just a bit frustrating because sometimes it works fine, and seems to be related to numbers, speeds, maybe even size of the zones. And it makes itself more evident in the long NAVC scenarios.
I completely missed those bug reports, I did look for ASW, sprint and drift, totally overlooking "patrol zone". Still there are 'workarounds' as the old noob Grumble points out, I for example often manually use fast subs to patrol ahead of the main formation, it requires a bit of micromanagement but it works. Or just moving around creep speed all the way long...
Maybe there are some magic formation numbers that work or other workarounds, if someone found or uses those, please point it out!
Thanks everyone, and hats off to Tony, meddling with decades old code must be, well, let's say INTERESTING.