I have noticed this as well. Each tick of a fire siege continues to do damage and pull you out of cloak. I have definitely observed with ranged fire siege, although I haven't tested specifically if it is happening with all fire siege (fire and coldfire ballista, fire and coldfire trebs, oil). I do not recall any patch notes saying this has been intentionally changed about cloak.
This was on a magicka nb with a significant magicka pool, PC NA, Vivec.
If this wasn't an intentional change, the thread should probably be moved to the bug reporting category.