I have tried this.
First by means of a soft(ware) System.reset() line in the code that was executed in case a Hardware Watchdog Reset was detected after start-ip. This did not work.
Then by driving a pin LOW that is externally connected to the RST pin of the Argon. Same result...
In both cases the system eventually locks up in dfu-like state (fast blinking yellow RGB LED).
The only way to get out of this state is Power Down -> Power Up.
It appears that the nRF52840 'somewhere' keeps track of the previous Hardware Watchdog reset action and does not clear this track when going through a pin (or system) reset.
Anyone there understanding what is really going on?