Can you show the parent process for each of the calls? Though I suspect now that one is udev while the other is systemd (because it seems that there's a systemd unit that will call mount-sd.sh, probably to workaround some issue like the one you have).
PID=224, ACTION=add ln -sf /dev/mmcblk1 /dev/sdcard ID_FS_TYPE=btrfs mkdir: cannot create directory `/run/user/100000': Permission denied mkdir -p /run/user/100000/media/sdcard, rc=1 home is mounted. mount -o subvol=.jolla/Desktop /dev/sdcard /home/nemo/Desktop/, rc=0 mount -o subvol=.jolla/Documents /dev/sdcard /home/nemo/Documents/, rc=0 mount -o subvol=.jolla/Downloads /dev/sdcard /home/nemo/Downloads/, rc=0 mount -o subvol=.jolla/Music /dev/sdcard /home/nemo/Music/, rc=0 mount -o subvol=.jolla/Pictures /dev/sdcard /home/nemo/Pictures/, rc=0 mount -o subvol=.jolla/Playlists /dev/sdcard /home/nemo/Playlists/, rc=0 mount -o subvol=.jolla/Public /dev/sdcard /home/nemo/Public/, rc=0 mount -o subvol=.jolla/Templates /dev/sdcard /home/nemo/Templates/, rc=0 mount -o subvol=.jolla/Videos /dev/sdcard /home/nemo/Videos/, rc=0 mount -o subvol=.android /dev/sdcard /data/sdcard/, rc=0 ---callstack--- UID PID PPID C STIME TTY TIME CMD root 158 1 16 15:44 ? 00:00:00 /lib/systemd/systemd-udevd root 163 158 4 15:44 ? S 0:00 /lib/systemd/systemd-udevd root 224 163 2 15:44 ? S 0:00 /bin/bash /usr/sbin/mount-sd.sh ---end--- PID=1318, ACTION=add ln -sf /dev/mmcblk1 /dev/sdcard ID_FS_TYPE=btrfs mkdir -p /run/user/100000/media/sdcard, rc=0 mount -o compress /dev/sdcard /run/user/100000/media/sdcard, rc=0 home is mounted. su nemo -c "ln -sf /run/user/100000/media/sdcard /home/nemo/MyDocs" ---callstack--- root 1 0 6 15:43 ? Ss 0:01 /sbin/init --unit=default.target root 1318 1 1 15:44 ? Ss 0:00 /bin/bash /usr/sbin/mount-sd.sh ---end---