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I'm not sure what the issue will be, but it certainly can be hard to keep a 2x8 joist straight and lumber quaility in a retail yard is terrible.The bigger the span of lumber the greater the chance of warping, bowing, crowning etc... if you get the chance, take a look at 4x4 and 2x2 pressure treated wood at lengths 8 feet and under and compare their quality to long lengths of 2x8. Compare pressure treated lumber that is incised with lumber that is not incised. Also note what grade all the lumber is; there's a big difference between select structural and No. 2.
I bought 2x2 in 36 inch lengths for a deck rail and waited 3 weeks before I was able to put it up, 2/3's of all of the pieces were warped. Had a similar experience with a fence and 4x4 pressure treated, 5 foot lengths all warped before getting them up. I also sold play sets and used premium grade pressure treated for those but dropped the line because of not being able to keep up with all of the replacement pieces on the sets. So no, I personally don’t believe that it’s a matter of quality, thickness or length. My experience is that it is an inherent flaw in all pressure treated wood to warp if it is not braced and/or bracketed in several places.
I guess you could say it is an "inherent flaw" in all wood to warp if it's not braced and/or bracketed and most importantly not properly stored, especially if the wood wasn't dried properly. Pressure treatment does require additional drying and storage consideration.
I'll post pictures of my wood framed tub also but it will be 3 years old soon, last time I looked it was as good as the day I bought it.