Note: I'm playing the Medieval Minecraft modpack on a server.

• Disconnecting and reconnecting makes you unable to retrieve the sword and it will pop back into your inventory after 20-30 minutes of gameplay.
• Traveling to the Aether biome made my Freyr disappear. It reappeared after about an hour, and poped back into my inventory after about 20-30 minutes.
• When the Freyr sword pops back to my inventory, it loses the enchantment I gave it, which really sucks as the sword won't be able to keep up with the mobs I have it fight against if I can't enchant it.

The first issue is "fixed". Now it will properly stand still when disconnecting and reconnecting instead of following the user even when the connection has been severed. Secondly, as of now, the Freyr Sword is unable to enter portals and travel through dimensions. If you want it to travel with you in other dimensions, you have to pick it up and place it there. I am guessing it popped into your inventory so long after because those dimensions it was previously in suddenly were rendered.
I'll see what I can do about enchantments being lost, will be fixed.