This was reported to my modpack over xJon/The-1.12.2-Pack#605.

After updating to the latest version of the modpack v1.3.1 from v1.3.0 when logging into my singleplayer world, my Armored Hardened Jetpack was replaced by a Dark Soularium Jetplate.

Any idea what caused this?

It happened to me as well when updating to 2.20.0, but that was all the way from 2.14.67.

This is due to the meta tags changing if an item is registered either before or after another item that is different to the previous version. Enjoy the free upgrade. 🌮

Sorry for really late resonse. Did this happen when updating to a even newer version, too? E.g. 2.19.x?
After a quick look it could have been related to changes in the recipe and Item "creation" in 2.18,x.