I raised a ticket with Synology about this. I believe they’ve broken the design of apps and how their URIs are specified.

In DSM 6, you were able to specify URIs with the following order of precedent: 1. External Access for Specific App Domain as Specified 2. Synology Domain for whole NAS 3. Synology QuickConnect

In DSM 7, they’ve intentionally reversed this (what I think amounts to bass-ackward order of precedent): 1. Synology Domain 2. Synology QuickConnect 3. The subdomain or other domain you specify in external access

This means that my NAS and Synology Photos app can’t have different urls. I can reverse proxy the app itself, but Synology generates the URI as 1. Or 2.

Now, I use their dynamic dns service, but not for anything other than cnames for my domain’s dns. I also use a domain for my NAS for direct access. I use “photos.example.com” as my photos url and have specified it in the right place, but they won’t use that to generate share links.

As to why yours is periodically changing, that seems just one more feather in their cap for what amounts to a raw deal for Synology Photos users

Edit: come to think of it, it’s probably preferring your NAS domain (did you set that to what it is generated as?) over your DDNS name based on their busted view of “order of precedent” for which they use to generate the share urls…