Navbar Breaks PHP redirect - php

Question

I have a navbar from bootstrap3.3. I also have various forms on this page which redirects with header location when a form is submitted. Whenever I add any sort of additional form or dropdown to my navbar(as opposed to the page body), the header location stops redirecting (the query still runs in the db as normal).
They should be completely independant of each other so I'm confused if this is a bug with bs 3.3? Or is my code flawed/have I made a stupid mistake?
form query works perfectly, but header location only if my navbar is relatively simple..
$result = $this->db_query($query);
if($result){
header("Location: https://somepage.com?message=submitted");
}
still works with this:
<nav class="navbar navbar-default">
<div class="container">
<div class="navbar-header">
<a class="navbar-brand" href="https://somepage.com">Brand</a>
</div>
<div id="navbar" class="navbar-collapse collapse">
<ul class="nav navbar-nav">
<li >Home</li>
<li>About</li>
<li>Contact</li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li>Order</li>
<li>Collections</li>
<li>Summary</li>
</ul>
</div><!--/.nav-collapse -->
</div>
</nav>
but won't work with this (tried form search and dropdown separately and together)
<nav class="navbar navbar-default">
<div class="container">
<div class="navbar-header">
<a class="navbar-brand" href="https://somepage.com">Brand</a>
</div>
<div id="navbar" class="navbar-collapse collapse">
<ul class="nav navbar-nav">
<li >Home</li>
<li>About</li>
<li>Contact</li>
<li class="dropdown">
Filter<span class="caret"></span>
<ul class="dropdown-menu" role="menu">
<li>test1</li>
<li>test2</li>
<li>test3</li>
</ul>
</li>
</ul>
<ul class="nav navbar-nav navbar-right">
<li><form class="form-inline" method="POST" action="">
<div class="form-group">
<input type="text" class="form-control" id="search" name="search" placeholder="Search">
</div>
<button type="submit" name="search" class="btn btn-secondary mb-2">Search</button>
</form></li>
<li>Order</li>
<li>Collections</li>
<li>Summary</li>
</ul>
</div><!--/.nav-collapse -->
</div>
</nav>