Okay, thanks.
If you have content in any other HTML Content area, move it into the main article editor.
The alignment issues are most likely being caused by WordPress automatically inserting break and paragraph tags where they are not wanted.
You can fix this by bringing your code for the form half of the tab all on to one line. Like this for example…
[one_half]<h1>Members' Sign In</h1><p>Sign in to begin browsing the members' areas and interacting with the community.</p><form class="publicForm" method="post" action="[fn function='wp_login_url']">[one_third]<div class="fieldContainer"><label for="log" class="formTitle">Username</label><input id="log" name="log" type="text" class="textInput" style="width:90%"></div>[/one_third][one_third]<div class="fieldContainer"><label for="pwd" class="formTitle">Password</label><input id="pwd" name="pwd" type="password" class="textInput" style="width:90%"></div>[/one_third][one_third]<button type="submit" class="btn black" style="margin-top:30px"><span>Sign in</span></button>[/one_third]</form>[clear]<br><br>[/one_half]
We’ll need to look into why the HTML Content area isn’t working in WordPress v4.9+. I’ll start a ticket for this with our development team and I’ll get back to you as soon as I know more. Thanks for pointing this out so we can get it fixed.