I will preface this with the fact that this testing was not done by me as my understanding of the game and how to test numbers is limited, but it has been tested and confirmed by one of the game's very best nerds.



How I have proved this:
Starting with a base character/no passives, equipping exclusively green frostbite, swift/mag reco jewelry, and no weapon. This means my only flat damage increases are the weapon damage lines, and the 570 max mag on the 2 body pieces. This means our relevant offensive stats for a skill are 1234 weapon damage and 12570 max mag (up from 1000, 12000).

Taking off 1 jewelry piece (which offers no offensive stats by itself), I can 1:1 compare the damage with and without the 5 piece. By using the damage calculation and Chilling Torch's damage coefficient (Using chilling torch because it has no skill rank modifier making for more accurate calculaton) we can predict that my Chilling Torch will do 964 damage to an 18200 armor enemy with 0 pen on me. This is correct.

By now re-equipping the jewelry piece to complete the 5 piece and dealing damage with Chilling Torch again, you can see I deal 1040 damage instead of 964. If we do 1.08* more damage in the Player % damage section of the calculation (where it would end up), this is also spitting out 1040 end damage, so the 8% works.

If we repeatedly spam Chilling Torch to get a Chill proc, which has a 5% base chance per hit of occuring (but likely lowered as this is a multi-hit skill lol), you will notice that there is no damage increase despite Frostbite's tooltip stating 4% should be added (additively, likely).

Next I tested Minor Brittle. By using Rune of the Colorless Pool (SoA skill line) I can guarantee it being applied, however this also applies minor vuln. I need to first test how much damage I deal with the same setup as previously, and then adding only minor vuln, which ends up as 1093 (which we can again verify using the damage calculation). I used the unmorphed skill for this, then I morphed it into giving brittle which would be the only difference. With brittle chilling torch did 1113 damage, which lines up with increasing 1.08 to 1.10.



Just to be overwhelmingly clear.
4piece Frostbite:
5piece Frostbite:
5piece frostbite with Chill proc (from 6-10seconds):
5piece Frostbite with Minor Vuln and Minor Brittle and Chilled:

This set has been out for a long time and this issue has been brought up on the forums and passed around via word of mouth for quite a while as well. I know the players who want to enjoy frost builds are a small but vocal minority, but I think the least we could expect is to have the sole good frost enabling set in the game at least work as intended. @BasP @ESO_Nightingale for your enjoyment as Warden enthusiasts.