Using secure pet template only works when solo. No units are shown when in a group or raid.

Are you setting the visibility correctly? I'm not able to re-create this.

I believe so. The raid frames share the same visibility.

visibility = 'raid,party,solo'

'showSolo', true,
'showParty', true,
'showRaid', true,

Then you'll want to write a small proof-of-concept. I tried with similar settings.

https://gist.github.com/705881/7b716b70ca10b44e2122e8b40513443861c93657

Managed to reproduce. Yet another Blizzard issue however. Tracking it down further later today.

Right now, you can workaround the issue by using 'useOwnerUnit' and setting unitsuffix to pet on the frames spawned.