The 6s did not meet FRA requirements on wheelset retention. They were operating under an FRA waiver and the waiver required use of nylon strapping to improve retention. The concern was that the wheelsets would come loose during a "high energy event" (translation: bad crash).
And some did break free in the Nisqually wreck. One of them nailed a car on I 5, IIRC.
NTSB investigation showed that the straps had never been inspected or replaced after initial installation. Testing showed they were only at 50% strength.
As part of their final report recommendations, the NTSB recommended immediate retirement Talgo 6s, due to that. It was found that the wheelsets coming loose directly contributed to the injuries and loss of life from the accident.
Washington DOT stated that they intended to follow the recommendations as quickly as they could. The service cutback caused by the pandemic allowed them to follow through on it.
As far as the bad blood goes, that was largely around the maintainence contracts that Talgo required. It boosted costs and Talgo was adamant about them. The wreck and the NTSB report did not help anything, but the issues between them were already there. Their immobility on the maintenance issue wound up pretty much costing them their North American market entirely.