I am saddened to report that am having the same issues as what is reported on the US Samsung community forums where the physical keyboard cover does not always stay on. Plastic film that was on rubber parts was removed as instructed before sticking it on. Often during extended periods of usage at certain angles the cover comes off (from the bottom long edge, if having the tablet in horizontal mode with keyboard out in front) :-(
Quite sad about this as item works well (when it stays on!) and is a compact elegant solution that also protects the tablet with little added bulk and don't have to remember to take along a separate keyboard everywhere (no key back lighting though). A large reason I considered switching from my iPad Pro to the Tab S6 is due to DEX mode, which needs a physical keyboard/mouse pointer IMHO to get the best out of.
Have asked the Amazon seller who I bought it from ("MoreFrom") for a refund.
And now my dilemma is, given this keyboard issue, should I return the tablet also... Hmm..
I don't see any reports about any other manufacturers creating an elegant portable keyboard solution for the Tab S6.
I had the same issue.
I made the mistake of wiping the "sticky" part of the cover with a damp cloth and drying it off with a towel, which made the problem immeasurably worse.
Then I discovered the solution, at least in my case!
I used a hair dryer to heat the inside of the cover (the part with the sticky pads). I didn't heat it up too much because I didn't want to warp or melt anything, but I probably spent a good 60 seconds passing the heat over the stick pads and in the middle where the vacuum should form.
I reapplied the case immediately to the back of the tablet, and then left it to cool back down to room temperature.
That was 3 days ago, and I've not had the back fall off at all since then.