do_shortcode causes headers to be sent -> pdf errror

hello
when calling this:

$sc = '[cfdb-value id="'. $form_id .'" show = "title" filter="submission_id=61"]';
$time =  do_shortcode($sc);

I get the expected data but headers are sent which breaks downloading the pdf i’m creating from the data. the do_shortcode() line is the problem.
This only happens when I’m logged in to wp.
is this a problem with wp or the plugin?
any way around it?
I’ve tried ob_start()… ob(clean() etc to no avail. Currently my workaround is to manually query the table

thanks