Tried to download trees for study ot_502 (which has two trees). The newick call obviously returned two trees:
curl https://api.opentreeoflife.org/v3/study/ot_502.tre/?tip_label=ot:ottid
but the Nexus call:
curl https://api.opentreeoflife.org/v3/study/ot_502.nex/?tip_label=ot:ottid
seemed to contain only one. Thanks to help by @kcranston it appears that the Nexus version is just missing a line break in between the two trees.