Why does bash send SIGTERM to program after logout despite setting nohup and disown

Question

I want to run a program which continues after logout from an ssh session.

I've tried:

nohup myProg > /tmp/logfile 2>&1 &
disown %1

and

systemd-run --scope --user myProg -p > /tmp/logfile 2>&1  &

In both cases, the process receives a SIGTERM from the bash process which launched it.

(The program uses sigaction for signal handling, which identifies the PID that sent the signal)

struct sigaction sigActionInfo;
sigActionInfo.sa_sigaction = DvsRebuildInfoFiles::signalHandler;
sigActionInfo.sa_flags = SA_SIGINFO;
sigemptyset(&sigActionInfo.sa_mask);
sigaction(SIGINT,  &sigActionInfo, 0);
sigaction(SIGTERM, &sigActionInfo, 0);

and

DvsRebuildInfoFiles::signalHandler(int sigNo, siginfo_t *sigInfo, void *context)
{
    pid_t sendingPid = sigInfo->si_pid;
    printf("DvsRebuildInfoFiles: Received Signal %d from process %d\n", sigNo, sendingPid);
}

Watching the log file from another terminal session confirms that it is bash which sends SIGTERM.

# ps -lf
F S UID          PID    PPID  C PRI  NI ADDR SZ WCHAN  STIME TTY          TIME CMD
0 S root      858355  858354  0  80   0 - 59839 -      03:40 pts/0    00:00:00 -bash
0 S root      858530  858355 12  80   0 - 944937 -     03:40 pts/0    00:00:01 ./DvsRebuildInfoFiles -p
0 S root      858906  858530  0  80   0 - 55622 -      03:40 pts/0    00:00:00 sh -c /usr/local/dvstor/bin64/DvGetProgramList /mnt/das.a/202307
0 D root      858907  858906  0  99  19 - 35888 -      03:40 pts/0    00:00:00 /usr/local/dvstor/bin64/DvGetProgramList /mnt/das.a/20230719.085
etc. etc. 

Why does it do that and can it be prevented?

NOTES

1. I don't know if this is relevant but the program itself launches many threads, each thread in turn executes another program via system().

2. Recoding the signal handler is not an option because the program must be able to be stopped with a signal.

3. /etc/systemd/logind.conf has the line commented out

4. Access is via ssh. No GUI.