The app overrides that. The address can be changed, but you need to do it in the app settings, not the Web GUI. Already fixed in the post above .
On a side note, in the Web GUI, you can use :8384 and it will automatically resolve to 0.0.0.0:8384, but in the app settings, you need to input 0.0.0.0:8384 explicitly. If you input just the port, the GUI will fail to load.
Thanks you @er-pa and @tomasz86 for the answers. Updating the configuration, changing the password and restarting worked.
The GUI is limited to 127.0.0.1 /localhost by default everywhere (see Getting Started — Syncthing documentation). This isn’t something specific to Android. Have you maybe encountered any third party installations/wrappers which used different settings?
Well, I was under the impression that my Linux devices (Arch, Debian, RaspberryOS) used a 0.0.0.0 setting out of the box. I am probably mistaken and memory fails me - I must have set that manually.
What is a bit surprising is that I would naturally have gone to my Android Syncthing settings to switch to 0.0.0.0 if I would have remembered that I did it elsewhere. It must be the age showing
I just checked and SyncTrazor is on 127.0.0.1 but I would not have tried to changed that anyway because it is my central machine for tinkering (i.e. I do not need to access its web interface remotely ever)
That as default would be very unsafe, especially on public Wi-Fi and similar . Syncthing actually displays a large warning message if you attempt to change the address to 0.0.0.0 with no username and password set in the GUI.
I did see that message on all my devices, including the Linux ones (and I disabled the message because it is not relevant in my case). If it appears after a manual override of a default 127.0.0.1 binding, it means that I indeed must have changed it on the Linux boxes and forgot about that.
I will be doing a DRP test of my home systems and services soon, I will test that comprehensively then.