Zyra's plants not working inside xin ult.

I'm almost certain it required you to be within the ult for them to work. Kind of like how basic abilities don't work on Xin if you're not in the ult for any other champion. Zyra's plants are no different, just because they have a health bar does not change anything.

Just to make it clearer: Xin's ult stops any damage from a champion outside of his zone (also minions / monsters). As such Zyra's plants are damage from Zyra, so if Zyra is outside of the zone the plants' damage is blocked (this is the logic that results in your case).

If however you can show that malz's w minions, Yorick's ghouls, Elise's spiderlings, heimer's turrets (idk about shaco's clone because it's supposed to mimic a champion) do in fact do damage whilst the champion is outside the zone then you can call this a bug.

Minions, Monsters, and Turrets will damage Xin even if they are outside his ult, but Pets / Clones / Traps / Plants / Bombs / Spells / DoTs / Delayed-Damage Abilities will not damage Xin as long as the source Champion is outside the Ult at the exact instance that the damage applies.