Why is the MSA showing on the Accounts Detail View?

When loading /index.php?module=Accounts&action=DetailView&record=<record_id>, instead of loading the full detail view, I get the MSA (twice):

 I've narrowed down at least one of the MSA's to be coming from header.tpl, but I don't know why it's being served literally instead of being parsed by Smarty. This is happening on 6.5.17 Professional. I fixed the issue once (not sure what I did - I tried running QRR, fixing permissions, & removing customizations), but it came back after the customer added a field to the layout and ran a QRR. Doing the same steps I did previously isn't fixing it this time. 

Any thoughts/advice/tips would be greatly appreciated!