Steps to reproduce
Make a spawn platform or enclosure to collect mobs.
Somewhere on the platform, set up a minecart next to a dispenser.
Put equipment in the dispenser.
Allow zombies to spawn naturally on the platform.
Push a zombie into the minecart.
Activate the dispenser, e.g. by placing a redstone torch next to it.
Send the equipped-zombie-in-minecart down a long minecart track, or fly away, so that it is beyond the instant despawn range.
Expected result
The zombie does not despawn.
Actual result
The zombie despawns.
Notes
This seems inconsistent with the fact that the same mobs would gain persistence by picking up dropped equipment.
On the other hand, strictly speaking this is consistent with the fact that mobs are supposed to gain persistence only from significant player interaction, and dispensers are not players. Compare MCPE-110521.
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Can reproduce