I made earlier a bug report thread on the same issue, yes the interface damage procs do not work properly at all.
People have also been reporting even the debuffs dont work as intended in most cases, one gentleman suggested reactives resistance proc didnt seem to work either, neither does the fire damage.
Yes, you are correct, in live the proc procced properly on ALL attacks you got, on ALL attacks pet did, etc. Currently it seems that from my pets (im a mastermind) procs dont work worth jackshit. If they do, they barely proc one damage tick (where using my normal powers, it seems that procs seem to actually tick more than once)
Interface SEEMS to work alot more reliably with direct damage your charachter is doing, pet powers that should do it 'supposedly' should do it as you get a random proc once in a thirty hits maybe to work when running +75 percent damage proc chance.
What reactive should do, is yeah that about 14 point damage tics x 4 for each procced damagesource you hit enemy with. If letsay you hit rapidly with 2 powers, they should stack so theyre burning for 14 x 8 (yet damage ticks would be overlapping)
Of course its possible devs decided to alter how interface works, if so it would be nice if we would be informed.