If you are running "motion" can't you have the Pi with the cameras ftp or scp the motion detected files to your server or send the telegram message using one of the motion command launchers like on_picture_save?
Personally I'd try to eliminate the server as it creates a single point of failure, unless you want a central archive of all motion detected images, I'm planning to use a router with a cell phone data plan as a fallback should the miscreants cut the internet connection, Right now I just send a message via a GSM modem if the internet or A/C power goes down. My UPS will keep the system up for about 70 minutes, This is the best I can come up with to eliminate my connection to the Internet as a single point of failure.
When another Pi puts pictures into the shared folder you are watching, is the user/group IDs the same along with the access permissions? There are some samba config options to force user/group and permission on the files coming it.
Is the node-red watch node using some kind of "hook" into the file system that writing the file as a share target doesn't trigger? Otherwise how does it know where the file comes from if its not a user/group permission issue?
Maybe you need to sync the filesystem after the transfer into the shared folder, I'd try an exec node in front of the watch to run the sync command and see if that helps.
Problem with timestamps as filenames is when the clock passes midnight. it changes the filename and usually the path so as to not over write files captured "yesterday" at the same time.