Internet Explorer SPDY ? Intermittent issues on 5.2.3?

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Bit of a weird one this.

Using Internet Explorer (11) on Windows 7, 8.1 and 10 and searching on https://www.google.co.uk from the internal LAN. Results are intermittently not returned, leaving an empty results page (with the terms completed in the search box but no results using the same search terms (see attached screenshot). 

Appliance is a 60D running 5.2.3

The relevant IPv4 policy implements SSL inspection using a self issued certificate, and AV, IPS, Application Control.

Nothing is logged in any traffic / UTM log to indicate blocked / restricted traffic for that policy / machine.

The same behaviour is not experienced in any other browser (Chrome, Firefox, Edge - all latest versions), nor is it experienced if we bypass the main firewall (via an unsecured guest network) or go through our test Fortigate 60D running 5.2.4 using an identical policy with SSLInspection and the same UTM limitations.

I did some searching on this yesterday and (at the moment) my best understanding is that it may be related to the IE implementation of SPDY. 

The FortiOS change logs indicate that SPDY was considered for Application Control policies as part of 5.2.4.

At the moment I'm working on the assumption that this will be resolved when we (in due course) upgrade the main firewall to 5.2.4. 

Is this a reasonable assumption?

In the meantime, is there anything I can do to workaround the issue on 5.2.3?