I don't think that sixth sense has changed at all.
*Prior to the new instant spotting mechanic introduced in 9.16, the amount of time that it took for your tank to get spotted was dependent on it's distance from the spotting tank. So it was possible to poke out and see/shoot an enemy tank, then duck behind cover before your tank was illuminated by his. So even though his tank did not illuminate you at that time......the system "saw your tank" and triggered the sixth sense timer. So once you got into the hiding the 3 seconds was over and your warning went off. This created the illusion that sixth sense was fast.
Now your tank is spotted instantly, and the sixth sense timer starts immediately. So this gives the enemy tanks the full 3 seconds to see and shoot you before the warning sounds. This is why you are experiencing your issue.
This new mechanic effectively rendered sixth sense useless IMO, and WG should either re-introduce a spotting delay, OR, shorten the delay timer on the sixth sense perk. Doesn't seem fair to destroy the functionality of a perk for which some players spent gold to acquire it more quickly.
*Based on my understanding of the game mechanics