I'm no expert, so this advice is worth as much as you paid for it
The version of Simpleviewer that ships with PM5 is quite old (v2.2 from 2011) the current version is 2.3.1. You could try downloading the
latest version and then replacing the old "svcore" folder (located at: /Applications/Photo Mechanic 5.app/Contents/builtin_templates/export/simpleviewer) with the new one. This
may provide a fix, or it may cause problems (I haven't tried it) so you may want to hold off in case far more knowledgeable people start screaming "NO!".
Simpleviewer's rollover to HTML5 version for mobile devices is a really nice feature, but it was never intended to provide the same look and functionality as the Flash version.
If HTML5 is really important to you, then I would look at
Juicebox from the same people. But you'd obviously lose the ability to use PM to export your galleries directly and would have to use their 'JuiceboxBuilder' app.
It may also be worth asking for advice on their
support forums.
As I say, I'm no expert so please tread very carefully.
Cheers,
Tony
NB: I've no links to the Simpleviewer folks, except as a purchaser of their 'Juicebox Pro' product.