Using Bard's ult on a tower that is not vulnerable to damage (say the 2nd tower in the lane while the 1st is still active)does not put the tower into stasis.
This is very easily to duplicate, though I'm not sure if it's an actual bug. It's unintuitive.
I'm pretty sure that structures with "protecting" structures still alive are actually untargetable and cannot be affected by anything.
The Wiki entry on Bard's page says:
Tempered Fate has no effect on untargetable towers (i.e. those protected by another tower or the inhibitors), untargetable units, or enemy champions that are immune to disables (such as OlafSquare.png Olaf under the effects of Ragnarok.png Ragnarok).
Seems intended.
I believe this was put in place to prevent an older bug where Bard could prevent a tower from being targetable by putting a tower into stasis as the one ahead of it was destroyed.
Bard's ult's intended to only hit things that are targetable. That means that champions can dodge its effects as a result by using Zhonya's or an ability like Vlad's W. Non vulnerable towers are also untargetable, so it doesn't affect them either (that wasn't the case for Bard's initial launch however).
