Armor Stands created before version 1.9 will have their items be in the wrong slots/deleted upon world upgrade.
Reproduction Steps
Create a world in 1.8.9
Place an armor stand
Give it arms with /entitydata @e[r=5] {ShowArms:1}
Give it full armor and place an item in its hands
Use this world where steps 1-4 are already done :
Load the world in the latest release
Observed Results
The armor stand now holds its boots and leggins in its hands (regardless of if the armor stand had arms before)
The item it had in hand is now in the boots slot
The helmet and chestplate are deleted
[media][media]Expected Results
The armor stand would be upgraded properly
Linked issues
Attachments
Comments 5

Can confirm on latest versions.
I request ownership because the owner is inactive and this report is quite ugly and needs to be made more concise

This is the same issue as MC-189525. Armor stands store equipment in the same way as mobs. It will probably also be fixed in the future version.

Now fixed in 25w02a thanks to the fix for MC-189525. This should likely be marked as a duplicate.
can confirm in 23w32a, the original issue is MC-200842 if you wondering. also here is the steps to reproduce:
1. create/open an 1.8 world
2. place down an armor stand
3. run this command: "/entitydata @e[r=N] {ShowArms:1}" (where N is a positive number)
4. place something in the armor stand hand
5. save that world and open it in 23w32a, you will see nothing in the arm