I'm not familiar with Synology's management layer for Docker, but your description almost reads as a similar observation for Portainer from about 2 years ago, where Portainer kept the container's previous environment (including version information) even when the container was recreated.
See Pihole Portainer/Docker 5.8.1 startup error? - #9 by ptruman.
Drawing an analogy from there, perhaps Synology has indeed pulled the latest image, but still shows the environment from the last run?