Event "SPELL_INTERRUPT" not firing

Hello, as the title says, I'm trying to implement in my UI (addon) a simple script that announces in Party when I get interrupted in Holy school.
With /eventtrace I can see the event type is "SPELL_INTERRUPT" but somehow it doesn't work when I write it into the addon.
It fires every other type of event (i.e. SPELL_AURA_APPLIED) but not the interrupt. I didn't even write the source/destsination/schooltype I just wrote something like

if eventtype == "SPELL_INTERRUPT"
then
print("Interrupted fired")
end

And it doesn't work, tested since hours with no results.

What am I missing? Thanks for reading.

EDIT: I made it work, now I can succesfully announce in party when I get kicked in Holy school. Problem is Penance, which is channeled, I don't see any "interrupt" in the /eventtrace, how can I track Penance interrupted? I see only something like "CAST_FAILED" but that fires even if you stop it (fake cast) so that doesn't help, I need the interrupt event. Thanks

Any help? :(