Up until this brake issue, their introduction had been going well.
I don't think that's true at all. TfW has missed several self-imposed milestones, is years behind schedule, and we know from insiders that there have been a significant number of issues behind the scenes.
I can't help feeling that TfW is simply overwhelmed and/or out of its depth, having introduced so many new types of unit at the same time, and has not been able to carry out adequate testing.
Let's not forget that these units were in scheduled passenger service for several weeks before they were withdrawn. We don't know what specific issue(s) led to the withdrawal, but they were clearly serious enough to cause the entire fleet to be taken out of service until further notice. Were passengers at risk?
Stadler delivered the first of a fleet of brand-new units to RhB (in Switzerland) in June 2019. They were introduced to passenger in service 12 months later, and have never had to be withdrawn on account of a technical defect or for any other reason. What is so different? Is it simply Swiss precision and attention to detail vs. the Welsh "oh, that'll do" attitude?