It seems that using the installation of syncthing via dietpi-software it create the configuration directory in /mnt/dietpi_userdata/syncthing/ but if the automatic updates integrated in syncthing are active the configuration is restored to its default directory /home/USER/.config/syncthing/ at first update. Maybe also with apt update?
This could lead to long-term data loss if you don’t notice the problem right away.
Stopping syncthing and moving /mnt/dietpi_userdata/syncthing/ to /home/USER/.config/ solve the issue, restoring shared folders and connected devices saving the user from troubles and reconfiguration.
It is certainly not a bug related to DietPi but you could move the default configuration directory set by dietpi-software or inform users from the syncthing wiki in https://dietpi.com/docs/software/cloud/#syncthing?
Thanks for your work.