Resolved! STM32U5 LSE/PC13 Errata and LSE bypass mode
The STM32U5 Errata Sheet (ES0499 rev 5 is the one I am looking at) has errata 2.2.1 which says: "The LSE frequency can be incorrect when PC13 is used as output for RTC_OUT1 or TAMP_OUT2 function". The workaround: "The RTC_OUT2 function must be used ...