We ran into this issue and the cause for us was our version of php (as well as php configuration) did not allow for <?= ?> syntax. Modifying the code to something like <?php echo $key; ?> resolves this issue (and is more likely to work across platforms).
NOTE: We literally just ran into this a few hours ago and were fortunate enough to this post. I’ll see if there is a more appropriate channel to log the fix.
