I've been able to replicate this several times in different worlds. The problem seems to occur when the allay are some distance from the nether portal (~70 blocks or more). After the player enters the nether and returns to the overworld, the behavior of the Allay change and they drift about aimlessly (like when they aren't holding an object). However, they ignore the leads until they drift far enough from the fence that the lead breaks. As @Spook_Lass mentioned, you can return them to their usual behavior by taking the item from them, hitting them and then giving the item back. Alternatively, if you reload the world, they will resume their normal behavior.
However, there is a second problem that sometimes occurs. When they break the leads, occasionally instead of dropping, it "stays attached" to the Allay. When this happens, you can't attach a new lead to them, since they think they are already attached to one. I have not found a way to fix this in the game, nor a definitive trigger for what causes it, but I suspect that the Allay object has some boolean value which indicates whether they are attached to a lead, and that value is somehow being corrupted.
I'm attaching three worlds here.
Allay Nether Test: This world has a group of Allay tethered 70+ blocks from a Nether portal. If you enter the portal and return, the Allay will exhibit the behavior.
Test Two Groups: Here there is a group of Allay tethered 70+ blocks from the portal, and another group tethered closer. The closer group won't exhibit the behavior, but the farther group will.
Allay Unleadable: There are two Allay following the player in this world which cannot have leads attached to them.
Haliaeetus
I've been able to replicate this several times in different worlds. The problem seems to occur when the allay are some distance from the nether portal (~70 blocks or more). After the player enters the nether and returns to the overworld, the behavior of the Allay change and they drift about aimlessly (like when they aren't holding an object). However, they ignore the leads until they drift far enough from the fence that the lead breaks. As @Spook_Lass mentioned, you can return them to their usual behavior by taking the item from them, hitting them and then giving the item back. Alternatively, if you reload the world, they will resume their normal behavior.
However, there is a second problem that sometimes occurs. When they break the leads, occasionally instead of dropping, it "stays attached" to the Allay. When this happens, you can't attach a new lead to them, since they think they are already attached to one. I have not found a way to fix this in the game, nor a definitive trigger for what causes it, but I suspect that the Allay object has some boolean value which indicates whether they are attached to a lead, and that value is somehow being corrupted.
I'm attaching three worlds here.
[media][media][media]Allay Nether Test: This world has a group of Allay tethered 70+ blocks from a Nether portal. If you enter the portal and return, the Allay will exhibit the behavior.
Test Two Groups: Here there is a group of Allay tethered 70+ blocks from the portal, and another group tethered closer. The closer group won't exhibit the behavior, but the farther group will.
Allay Unleadable: There are two Allay following the player in this world which cannot have leads attached to them.