Moisture did not cause accumulators to wear out. They simply fail from age and internal flexing. Please go to my website 'activesuspensionsystems.com' and read all about it. At the Dealer expect to pay between $3500 and $4000 for the parts and labor. From me, a better part , 3 times the warranty, DIY for $1400 total. Labor to install by a mechanic 1 hour total.
At 61k the dealer did the big 60k service. The problem with the bouncing began around 62 k. At 64k the dealer changed the Height control Oil because they said it was contaminated. Will a contaminated hyd system caouse these accumulators to fail?
The dealer goofed... Make them fix it. If they give you any pushback call Lexus and explain your situation. I am sure Lexus will step up.
Good Luck!