Thank you for the reply. Based on this post:
I thought that flag would clear any vfs cache on the system on start of the mount. Is this not the case?
Wrt to debug log I can try to capture but as I noted it somewhat random and difficult to reproduce.
Systemd service:
Description=backup Daemon
After=multi-user.target
[Service]
Type=notify
ExecStart=/usr/bin/rclone mount /mnt/src /mnt/dst \
--uid=1000 --gid=1000 --umask=002 \
--allow-other \
--read-only \
--timeout=1h \
--dir-cache-time=120h \
--attr-timeout 120h \
--vfs-cache-max-age=4h \
--vfs-cache-min-free-space=80G \
--vfs-read-chunk-size-limit=128M \
--vfs-read-chunk-size=8M \
--vfs-fast-fingerprint \
--vfs-cache-mode=full \
--links \
--cache-db-purge
ExecStop=/bin/fusermount -uz /mnt/dst > /dev/null
TimeoutSec=120
Restart=always
KillMode=process
User=0
Group=0
[Install]
WantedBy=multi-user.target```