Theorem and Demulcent arcanes won't work on Yareli if she is riding her Merulina.

They will resume working once you leave Merulina, but won't work if you are riding it, most Squad buffs like Roar and Eclipse won't work either.

submitted by /u/mokousama
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source https://www.reddit.com/r/Warframe/comments/x6uzd1/theorem_and_demulcent_arcanes_wont_work_on_yareli/