I did some tests with Crown of Tempests and Attunement of Conduction. It seems like CoT isn't extending the duration of Stormtrance at all. These are the numbers I found. Did the test multiple times and they were consistent. All tests were done with Attunement of Conduction active.
ST = Stormtrance
CoT = Crown of Tempests
A = Abilities (- A means on cooldown, + A means off cooldown)
ST + CoT + A = 22 seconds
ST + CoT - A = 17 seconds
ST - CoT + A = 22 seconds
ST - CoT - A = 17 seconds
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Did you have stacks with crown of tempests? Are you able to provide a video for reference?