Current Behavior:
When a dispenser with an armor stand in it is powered the dispenser ejects the armor stand as an item.

Expected Behavior:
When powering a dispenser with armor stand in it the expected behavior is for it to be placed on the block in front of the dispenser.

This issue was accepted as a bug in Minecraft Java. It was solved in https://bugs.mojang.com/browse/MC-131061 and announced as a fixed bug in 19w42: https://www.minecraft.net/en-us/article/minecraft-snapshot-19w42a