Known issue, reported in Hang Starting nRF52840 Feather Express Under Segger Debugger - #8 by geeksville with workaround described in Noob JLink Edu + Arduino Nano 33 IOT + PIO 5.01 Debugger woes – exact cause still unknown, though. Speculations say it’s something to do with the bootloader or the USB stack. Also I think someone talked about a easier workaround, just hitting the “Restart” icon in the debug bar.