Q: Why isn't my Kaedehara Kazuha getting the Elemental Mastery bonus from Sucrose?
A: In order to avoid infinite stacking of in-game bonuses, when an effect confers one attribute as a certain percentage of another attribute, this effect will not then factor into calculations of other similar percentage-based effects. We included an explanation regarding this issue in the loading screens in Version 1.3.
For example, Sucrose's Ascension Talent “Mollis Favonius” grants all characters in the party (except Sucrose) an Elemental Mastery bonus equal to 20% of Sucrose's Elemental Mastery. This is an example of an effect that confers one attribute as a certain percentage of another attribute. Kaedehara Kazuha's Ascension Talent "Poetics of Fuubutsu" grants all characters in the party an Elemental DMG bonus proportional to his Elemental Mastery. The Elemental Mastery value used to calculate this Elemental DMG bonus does not take into account the Elemental Mastery increase granted by Sucrose's Ascension Talent.
For example, in a party whereby both Kazuha and Sucrose have 100 points of Elemental Mastery each, Sucrose takes advantage of her Talent, thus granting Kazuha an Elemental Mastery bonus of 100 × 20% = 20 points. Kazuha's Elemental Mastery therefore increases to 120. But if Kazuha triggers a Swirl reaction, which in return activates his Talent, he obtains an Elemental DMG bonus of 4% (and not 4.8 %).
Without this restriction, if we added another effect in the future that would read something to the effect of "the entire party's Elemental Mastery increases in proportion to the target's Elemental DMG bonus," or "the entire party's Elemental Mastery increases in proportion to the target's Elemental Mastery," the effects would stack up infinitely.
It is also worth mentioning that bonus effects conferred by characters or equipment that directly increase attributes by a certain value (as opposed to a percentage), such as the effect granted by four pieces of the "Instructor" artifact set, Albedo's Ascension Talent "Homuncular Nature," and the effect triggered by the catalyst The Widsith are not affected by this restriction.