Hopefully someone with a V2 systematic will weigh in on this. I would assume nothing is changed with the latest V2 models unless you hear otherwise from an owner.
A Mountaineer will not do that and can not do that because of a protuberance below the mount that stops the leg from going much further than the normal folded tight position. My older G1228 (predecessor to GT2541) is built this way and I assume all Mountaineers are built this way, including the Series 3.
With a center column installed there is even more obstruction to prevent that, but even with the center column removed the leg will only go slightly past "vertical".
I have two Systematics... G1410, an older alloy Series 4 and a GT3541LS.
Both systematics will do what you describe. My sense is that it is much more of a problem if the legs are not tensioned as tight as I tension them.
I have had this happen, more or less, a couple of times, I think, walking trails with my 500/4 and Wimberly in the conventional over the shoulder carry.
My sense is that it is something that is potentially problematic because it can cause a weight shift that could conceivably get out of hand. Just one reason why the standard conservative advice is to do what few of us do- always remove gear before transporting the tripod.
I am conscious of this and try to make sure the legs are folded in tight such that each pair of legs keeps the 3rd leg from swinging through.
I have had a different problem when carrying the rig upright, as in my avatar, but in that case it is a leg fully opened that collapses toward the center (but never gets near going through the middle- although it conceivably could). That can cause a very dangerous weight shift if you are not properly balanced to begin with.
I have never considered this a deal breaking issue. I consider it just something that you learn when you start to deal with handling the tripod however you elect to carry it, especially with gear attached. There is a bit of an art and hopefully it is not learned the hard way.
I would like to give you a simple up or down on this but I cannot. It's a grey area, I guess, a glass half empty or half full issue.
I have no doubt that keeping good solid tension on the legs eliminates much of this. My legs need some effort to open. I do that for reasons of stability, not necessarily to deal with this issue, which I rarely think about or experience.
I have never had a problem, with any of my 3 Gitzos, with the legs loosening over time. Some have gone years, probably, between adjustments because they just stay tensioned right. Others have different experiences but that is mine. Each time you open or close the tripod you should be at least vaguely aware of the tension (or lack of) and adjust if needed.