I can imagine very frustrating on a long trip. (Talon IIRC?)
Luckily mine only came off on the penultimate day of our week in Assynt/Sutherland last year. I knew about them being a pain, but the rest of the Pack's features ticked the boxes. And, it was half price. It's a great carry though it swallows my gear. I got it for winter trips really.
Tempted to bore/drill a few holes through each plastic slider piece and stitch it to the "piping" so it can't come off again. I don't need to adjust it at all for my usage, and the threads can be cut by another user if necessary.
Bout 11yrs back the sternum strap in my pod alpine 40 came off, I tried the above method with a coin and snapped it. Wrote to Equip outddoors tec uk, told them it came off in use and snapped when I tried to put it back on, and could they send me a spare that I'd be happy to pay for.
They wrote back telling me to send the pack back for them to fix, I stripped it and did.
I now have a alpine 40 and 2 lids, 2 hip belts and 2 removeable stay sheets as they sent me out a complete new pack.
I have a crag sac size 3 too (as well as 3 of the lux models), which I think is 47l, it has a narrower and longer stay sheet than the alpine. Don't know what model you have but I think the Black ice is a 50l, so maybe taller again.