The joy of outsourcing
Postbank's IT was managed in-house by the 100% owned Postbank Systems. But in 2020, Deutsche Bank as owner of PB, and in yet another one of their questionable initiatives (see DB's history), sold PB Systems to the Tata Group(*). Nothing wrong with that - staff from Tata and other Indian consulting companies had been running around the Dark Tower of Mordor (DB data and IT centre near Frankfurt) long before the sale. At the same time, various efforts to integrate or migrate PB's IT into DB's systems did not always work out quite as planned...
But whether staff at PB Systems, many of whom had been working there for many years and knew their systems in and out, were really that keen on being shunted off to a new employer? And so, out-the-door the knowledge walked.
Correction to the cited FT article: In 2010 Deutsche Bank did not have full control as it was waiting for regulatory approval to take over the then still "independent" Postbank. I believe approval came later, probably in 2012, (can't be bothered to look it up). It was a funny situation - everybody knew DB would be the new master but officially, PB was still operating and deciding autonomously, with "delicate" interactions between the two organisations, so as not to piss off the regulatory bodies.
* https://www.db.com/news/detail/20201109-deutsche-bank-announces-sale-of-postbank-systems-to-tata-consultancy-services?language_id=1