There are several things (some of them you already identified) that cause the front foot to come out of the front footstrap.
It could be your stance (I'd like a photo of you at full speed to really identify any stance problems).
Boom height can have a large impact on the weight (or negative weight) on your front foot in the footstrap.
Are you trying to keep weight on your front foot, or are you lifting slightly....?
Harness line length also can affect the + or - pressure on your front foot.
Footstrap spacing and position can have a large effect as well......where do you
have your footstraps placed on the board?
What fin are you using?
How you have your RS 6.7 m2 slalom sail rigged (too full maybe)? This could have alot of impact on your foot pressure.
Is your rig pretty much straight up, or are you "hooking it" slightly upwind (a little too much upwind if it's trying to pull you up off the board).
The Isonics (I haven't sailed one as small as the 86 liter so I'm basing this on larger 101 liter and 122 liter Isonics) like a nearly vertical stance (not leaning back too far) in the conventional "7" configuration.
Perhaps you are actually standing up too tall (hence very little side pressure on the footstrap to keep your foot in).
Try your boom at the absolute bottom of the boom cutout in your sail.
High booms are great for Formula racers, but slalom speedsters tend to run their booms
quite a bit lower. Just drop it way down and see how it feels, then move it back up incrementally until it feels the best.
Keep adjusting your boom height, shortening/lengthening your harness lines (trapeze),
and adjusting your stance until everything begins to balance.
Then "fine tune" to get that really perfect balance and mark all your settings so you can easily duplicate them on your next session on the water and use the previous "best balance" as your starting point next session.
Also, only make one change at a time.
Often we change the boom height, trapeze, and mast foot position at the same time thinking we can achieve a better "balance" but too many changes just gives a different
(not better) balance and it still feels out of whack.
Without seeing your stance, and perhaps sailing your rig, it's very hard to tell which of the many variables here is causing your problem.
Hope this helps,
P.S. one of the best ways to "balance" things out on the water is to get a set of good adjustable harness (trapeze) lines, so you can shorten/lengthen your lines and adjust your stance while at full speed (well, maybe not full speed, but something close).
Adjusting anything, at full speed, overpowered, in chop often means you go swimming/get wet/wipeout..... again!