I'm having exactly the same issue—this must be a change in 12.0.3, as my solution worked fine until very recently (Easter time was the last time I tried it). Basically, the page never refreshes, but 'hangs' at the form submission page. The same page in Safari and Chrome works fine.
I don't think I ever solved the web viewer problem, and, in this particular case, had to go outside filemaker. However, there are some scripts that I use to get web page content thatlike to call from within filemaker that might be made to work, namely using applescript to call a curl request:
Perform AppleScrfipt ["set cmd to \"curl 'http://yoursite.com/somepage.php?var1=val1&fmvar1=" & FilemakerDB::Field & "'\"" & ¶ & "set applescriptVar to do shell script cmd" & ¶ & "set cell \"FilemakerDBField\" of current record to applescriptVar"]
Should be more flexible anyway.
I'm experimenting with adding a flush(); somewhere early in my php script, to let the webviewer know that something is on the way back.
Very irritating, though. Even if it is going to time-out (without the ability to adjust the timeout length), they could at least show a 'page not available page' to let you know what's going on.
I can work around by opening a browser, but it's a lot less elegant.