I think this is STM32U5 specific. ST adds "Resource isolation improvements" on top of the ARMv8-M TrustZone. These improvements allow a block-based secure and privilege configuration for internal SRAM.See page 8 on the STM32U5-Security-Overview (SECO...

I had exactly the same problem.In my case it works now. The default settings in the SRAM3 GTZC1_MPCBB3_PRIVCFGRx configuration registers allow only privileged access (with enabled TrustZone). After setting these configuration registers to 0 it works.