I don’t understand what’s happening either. I suspect like you that it’s a timing issue.
Here is another idea, instead of using fstab mounts switch over to using autofs mounts. I have found that autofs works much better if you have systems that are not always online vs systemd mounts.
Have a look here to get started: Mounting network shares with autofs (alternative to fstab)
An example of what a autofs map would look like:
/mnt/sshfsmount -fstype=fuse,rw,uid=1000,gid=1000,allow_other,identityfile=/home/osmc/.ssh/id_rsa :sshfs\#remoteuser@remote.ip.address\:
The backslashes and # are not mistakes, they are needed. I use this on my laptop and it works fine. I never really used the fstab version other than for testing for other users and occasional manual mounting. With autofs it just mounts when I need it.