Jukeboxes that have been placed don't stack with jukeboxes that have not been placed

can confirm for non-optifine 1.16.5  

@Adan Chappuis Ideally remove the 'optifine' bit

I've tested this in vanilla single player in both 1.16.5 as well as the latest 21w07a. It still happens exactly as described, although it apparently has nothing to do with optifine so I removed that part.

In the setPlacedBy method of net.minecraft.world.level.block.JukeboxBlock (mojang mappings),

an empty compound tag is created on the item stack that is used to place the jukebox.

Can confirm in 21w08a.

Can confirm in 21w13a.

Cannot replicate in 21w37a and further versions.

This has been fixed in 1.18

Can confirm that this issue has been fixed.

This issue seems to have been introduced in 1.16-pre5 when MC-180867 was fixed. That version added the following code to JukeboxBlock.setPlacedBy (as noted by @unknown):

CompoundTag tag = stack.getOrCreateTag();
if (tag.contains("BlockEntityTag")) {
  CompoundTag tag2 = tag.getCompound("BlockEntityTag");
  if (tag.contains("RecordItem")) {
    // Update has_record state
  }
}

21w37a replaced that code with this:

CompoundTag tag = item.getBlockEntityData(stack);
// above is equivalent to stack.getTagElement("BlockEntityTag"), which doesn't create tag if there is no tag
if (tag != null && tag.contains("RecordItem")) {
  // Update has_record state
}

I also am able to reproduce the issue in 1.17.1 but not in 21w37a, so I'm marking it as fixed. (MC-180867 also remains fixed.)

It doesnt even contain any extra nbt data, so Luckily this seems to be solved by throwing the items on the ground, then they can both be picked up and stacked as normal