I wrote that if you upgrade a weapon to level 4 or 5 in the game, its level in SOUL FORGE will be shown as one level lower. There might also be a reverse bug.
I will be able to check this in 6 days. I currently have the Staff of Tides being upgraded to level 5. If the ice barrier is not applied, then upgrading to level 5 in SOUL FORGE is not desirable; it is better to upgrade it to level 4 and then do it yourself.

Added 18 minutes later
Oops, I was wrong; the Staff of Tides was actually upgraded to level 5.

The result is that there is NO ICE BARRIER, meaning that SOUL FORGE does not actually upgrade to level 5 (although it shows a full upgrade), but only spoils the weapon, as it deprives you of the ability to upgrade it yourself through the game.

Added 58 seconds later
If SOUL FORGE is currently upgrading a weapon to level 5, play at least one battle with that weapon in the game, and then it will be removed from the forge.