I did some more testing, and as far as I can tell the passive (1) only affects max magicka, not stamina, and (2) doesn't activate when changing armory slots, a barswap after seems to trigger it. Once active it seems to remain active, even after relogging.

Same problem. I also tried dismissing any companions and non-combat pets, unslotting all Daedric Summoning abilities, heading into a dungeon, getting into combat, and bar-swapping. None of them worked for me, my magicka and stamina kept the same unbuffed values.

The unchanging Stamina and Magicka value you see when summoning a pet is the amount you'd have from the bonus for not having a pet. In other words, your actual Magicka and Stamina values are lower than what you see when you have a pet active — the values just don't update in a timely manner.

To illustrate this, first note how the value for Maximum Health takes a while to update on the character sheet:


The Maximum Health value goes from 39346 to 42859, which is approximately an 8.9% increase.

Then, to refresh the frozen values for Maximum Stamina and Maximum Magicka, you can do something like update your attributes:


Notice how the Stamina and Magicka values update, despite the change in attributes only affecting Health. For instance, Maximum Magicka drops from 20678 to 19011, which is approximately an 8.7% difference. Likewise, Maximum Stamina drops from 34909 to 31900, which is approximately a 9.4% difference.

While I'm not sure why these values aren't exactly a 10% difference, I'd wager that it has something to do with other passives affecting Maximum Stamina/Magicka/Health.

You can also verify that the stats are being given by having a pet summoned, then learning the Expert Summoner passive — only the Health value changes. Likewise, learning the Expert Summoner passive without pets summoned will only change the Stamina and Magicka values.


TL;DR: The Magicka/Stamina value you have before learning the Expert Summoner passive is how much you actually have when you have a pet summoned with the passive learned. Because you have less Magicka/Stamina before learning the passive than you do when you have the passive with a pet summoned, the values are not properly updating, leading people to think that they aren't getting the bonus to Magicka/Stamina that they already have.