... for "S&G", I tried logging into yamahasynth.com's forums using Safari on an iOS device running iOS 12.0.1. The Safari version is 12.0 (latest for iOS). What I saw was the login did not work. Steps to reproduce:
1) Login to
http://www.yamahasynth.com using Safari on above detailed iOS device - this brings me to the home page and I have never signed in before (no cached credentials)
2) Touched the "Sign In" button
3) Filled in my credentials using the yamahasynth credentials (not facebook, not google). - Site accepted my username and password. I let the redirect (back to the homepage) happen by waiting instead of pressing "continue".
4) Back at the homescreen I saw the "Sign In" button has properly changed to the "My Account" and "Sign Out" buttons. Going by memory on "Sign Out" - so that's a paraphrase.
5) Navigated to the Forum by pressing the hamburger menu (3 horizontal lines like "=" is 2 horizontal lines), touched "Forum", then touched the Non-Synth area to see posts here
*** Unexpected behavior w/Safari: ***
6) Noticed that I could not add to a post - even though the top had a "My Account" button (recognizing I have logged in, because I could touch it and see my name) the bottom of the screen showed the different way of logging in with a text box for username and a text box for password instructing me to login in order to reply.
7) I could, however, fill in THIS method of logging in and then the system would allow for me to add a reply.
I would agree that iOS Safari ("Mobile 12.0" ) - right now, today 11/16/2018 - presents a different experience than I see using Chrome on Windows. And I can say that this mis-alignment between the "Sign In" button and other methods of entering user credentials was once a problem even with Chrome and that once I would see differences between Chrome/Firefox/Opera under Windows which had me, in the past, trying all 3 browsers every time I tried to login. Now this is fixed for the PC - but Safari seems to have some of the same old behavior I saw before.
For me, the workaround is to login using credentials at the bottom of a message post - as this method seems to work. And NOT to use the "Sign In" button at the top of the screen (banner area). It would probably serve to get this fixed for Safari and perhaps regression test other browsers and OS environments.