If I manually stop my service and then execute echo V > /dev/watchdog1, the watchdog stops properly.
If I do the same echo command in my systemd service, I get:
watchdog did not stop!
ExecStopPost=echo V > /dev/watchdog1
Why is the behavior not the same?
This does not work for the same reason mentioned in this post: Execute multiple commands with && in systemd service ExecStart on RedHat 7.9
The commands from inside a systemd service are not executed in a proper shell environment. Even so, I do not have some kind of source that explicitly states this. From experience, the capabilities of a single systemd exec are the following: Run one command with parameters (not multiple commands, no output redirection, etc.).
Just like in the referenced post, the solution could be writing it as follows:
ExecStopPost=/bin/bash -c 'echo V > /dev/watchdog1'