Let $E/F$ be a quadratic extension of number fields and let $pi$ be an $mathrm{SL}_n(mathbb{A}_F)$-distinguished cuspidal automorphic representation of $mathrm{SL}_n(mathbb{A}_E)$. Using an unfolding argument, we prove that an element of the $mathrm{L}$-packet of $pi$ is distinguished if and only if it is $psi$-generic for a non-degenerate character $psi$ of $N_n(mathbb{A}_E)$ trivial on $N_n(E+mathbb{A}_F)$, where $N_n$ is the group of unipotent upper triangular matrices of $mathrm{SL}_n$. We then use this result to analyze the non-vanishing of the period integral on different realizations of a distinguished cuspidal automorphic representation of $mathrm{SL}_n(mathbb{A}_E)$ with multiplicity $> 1$, and show that in general some canonical copies of a distinguished representation inside different $mathrm{L}$-packets can have vanishing period. We also construct examples of everywhere locally distinguished representations of $mathrm{SL}_n(mathbb{A}_E)$ the $mathrm{L}$-packets of which do not contain any distinguished representation.
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