Feeding Frenzy Support does not apply increased duration from the passive tree

I've never used the Feeding Frenzy Support gem prior to today so I don't know for how long this has been an issue, but today while experimenting on it I noticed a discrepancy between the buff duration shown on the tooltip and the actual buff duration as applied to the player.

Using a lv 12 Feeding Frenzy Support with 5.1 second duration on a Carrion Golem, I've noted the following. 105% increased duration from the passive tree applied to the 5.1 second base amount comes out to 10.455 seconds, or 10.5 seconds as reported in the tooltip. When I actually receive that buff from my minion, it's only about 5 seconds, the base amount. If I add a level 1 Increased Duration Support gem with a 45% increase, the total increased duration comes out to 150%. When applied to the 5.1 second Feeding Frenzy base duration, it should be 12.75 seconds, or 12.8 seconds as reported in the tooltip for Summon Carrion Golem. The duration from the buff I actually receive is about 8 seconds, because it's only taking the 45% increase from Increased Duration Support and ignoring the passive tree entirely. If I deallocate the skill duration nodes from the passive tree, while keeping the 45% Increased Duration Support gem, I expect to get 7.395 seconds of duration. The tooltip rounds this to 7.4 seconds and when I get the buff, its duration turns out to be that much.

To summarize, based on my testing, Feeding Frenzy Support's buff duration ignores increased duration from the passive tree, but the tooltip shows what should be the correct duration.

I made a video to demonstrate the issue: https://youtu.be/lytO1Zb3cqc

gem's duration is internal & cannot be modified outside of increasing its quality.

Then why would the tooltip suggest otherwise, and why would the Increased Duration Support be able to modify it and not the passive skill tree. This response makes no sense to me. All my other non-ascendancy buffs appear to increase their duration from passives on the skill tree, as expected, but not Feeding Frenzy.