Intermittenly blocked by application filter

Hi,

I've got a site that's being blocked by the student policy
(yearbookavenue.jostens.com 192.189.112.187). It's a big single page web
app type site that makes a lot of xml http requests. Students can visit
the site just fine, but when students go to save their progress on the
site it makes a POST request to yearbookavenue.jostens.com/savestuff or
whatever and this will work fine on my machine when I'm hitting the
Staff policy, but produces an error when on a student account on a chromebook.

Looking in the firewall logs I can see that the ip address
192.189.112.197 is allowed when the "Application Name" is HTTPS.BROWSER but blocked when it's SSL. I've
allowed the site in web filter but it seems like it's still being
blocked at the application level somehow, like the SPA is producing a
different application signature. To make things trickier sometimes traffic to that site produces HTTPS.BROWSER when student access it, allowing them to save, but other times it produces SSL traffic and it's blocked.

I've tried adding an application signature

config application custom
edit "yearbookavenue"
set comment "yearbookavenue.jostens.com signature"
set signature "F-SBID( --attack_id 6694; --name Allow.YearbookAvenue.jostens.com; --pattern yearbookavenue.jostens.com; --service SSL; --protocol tcp; --no_case; --app_cat 32; )"
set category 32
next
end

This is my first time writing one and I'm kind of confused as to how it works, and I've also failed to apply it to the student application policy.

I'm therefore kind of stuck on how to allow yearbookavenue.jostens.com
on the fortigate. I've included a screenshot that can show you what I'm
talking about a little better. I filtered only destination
192.189.112.187 and you can see that SSL is getting denied and
HTTPS.BROWSER is allowed.

Thanks,

Jordan