Resolved! STM32U575 is memory remapping required - prior to jump to bootloader from application?
In an earlier project based on STM32F7x, we had to add memory remapping by hand:SYSCFG->MEMRMP = 0x01;There is no equivalent in SYSCFG for STMU575, nor did I find relevant bits under SYSCFG->CFGR1 or CFGR1->CFGR2 or other field as part of SYSCFG.