I didn't know this myself but I then experienced it for myself awhile back and I can confirm with first hand experience that it does not wear off when the THF dies. If it's ever happened it probably got bugged in that instance or people assume too much, that they got a proc in before they died and then see a proc land and think, "oh, it must have worn off and started over".

I'm also pretty sure but not 100% that I've had some cases where I wiped and as I was spacing out figuring out what went wrong I noticed TH effectiveness wearing off after a minute or so. I can't be sure on that because it's pretty rare for me to die ever, and I'm pretty sure the last time I died on THF was months ago and was when I claim to have seen TH effectiveness wearing off like any other debuff that's applied and wears off after a wipe. Not that it matters really but if you need more weight for the "no" answer...

If you just gotta see the post they made then I suggest clicking on the Dev Tracker. It lists every post by the reps, I figure. But, maybe not.