Stable application crashes at address 0x00000000 when I add JLINK_MONITOR_ISR_SES.s but DebugMonitor_IRQn is disabled

Question

Very strange, I have lots of nRF52x experience, but this one has me stumped. 
 Successfully updated our custom board/application from nRF5_SDK_15.0.0 to latest as indicated below: 
 Setups: 
 SDK: nRF5_SDK_16.0.0_98a08e2 
 SoftDevice: 
 FreeRTOS (from SDK) 
 Segger Embedded Studio (same issue on 4.12 / 4.18 / 4.50) 
 Windows 10 
 Custom board 
 
 Preprocessor definitions: 
 BOARD_CUSTOM 
 
 NRF52 
 NRF52832_XXAA 
 NRF52_PAN_74 
 S132 
 SOFTDEVICE_PRESENT 
 NRF_SD_BLE_API_VERSION=7 
 CONFIG_NFCT_PINS_AS_GPIOS 
 FLOAT_ABI_HARD 
 INITIALIZE_USER_SECTIONS 
 NO_VTOR_CONFIG 
 ARM_MATH_CM4 
 FREERTOS 
 
 After migrating to SKD16, everything works very nicely. It is a reasonably complex application. 
 We use default flash_placement.xml from SDK16 example applications. 
 Application passes all of our testing. In Debug mode, NRF_LOG via RTT is active. This all operates fine. 
 Verified we have excess RAM available to the application. RAM_START=0x20002a98. 
 ret_code_t ret_code = sd_ble_enable(p_app_ram_start); returns a start value of 0x20002220 ... so we are good here. 
 To help me isolate the problem, I have erased my nRF52832 IC, and no longer are using our Bootloader. 
 Only the Softdevice and Application are being loaded via SES. 
 
 The last to do item was to re-enable our JLINK debug monitor. This is the ONLY change we are making to the application 
 We use the recommended method, as outlined in https://github.com/NordicPlayground/j-link-monitoring-mode-debugging . We use the 3 files as is unmodified. 
 We use the SDK16.0.0 branch, and our JLINK options are set to: 
 SetMonModeDebug = 1 
 SetMonModeVTableAddr = 0x26000 
 We execute NVIC_SetPriority(DebugMonitor_IRQn, _PRIO_SD_LOW); in our Logger task. 
 When we do this, our application does not startup up at all, and the SES Debugger shows us at address 0x00000000 ... see attached picture below [Crash.png] 
 Note also the messaging indicating: T-bit of XPSR is 0 but should be 1. Changed to 1. 
 
 To further isolate the problem, I commented out NVIC_SetPriority(DebugMonitor_IRQn, _PRIO_SD_LOW); ... but no change. ..

Very weird ... [DebugMon_Handler] is being compiled in, but not being executed. 
 ... and all of this causes the SES to fail in loading the application. I am a bit stymied. 
 I tried setting early breakpoints in thumb_crt0.s ... but it would not hit any of the early breakpoints. 
 
 As a very last investigative step, I took the SDK 16.0.0 example application, and modified it to support JLINK_MONITOR ... 
 ... after some tweaking ... works like a charm :} 
 
 Free beer to whoever can solve my problem :}