So, short fix is download the faces while you can and save them local until Facer changes the way this app functions. Not really Facers fault, more of a watch issue.
Write back if this this not clear 
I have been having this problem for some time and I now have the app working as it is supposed to.
On my S8 I went to settings > apps >facer and pressed ‘Force Stop’.
I reopened the app and I was successfully connected to my Gear S3. I changed the watch face several times without issue.
It’s worth a try (WOM)