Reproduction:

• Create a new superflat world with creative and cheats.
• Disable entity drops (/gamerule doEntityDrops false)
• Spawn a pig and a fence.
• Attach the pig to the fence.
• Break the fence.
• The lead will drop.

Observations:

Itemframes will not drop the item when you remove the supporting block.

However, the lead will drop when you break the fence.

Conclusion:

Clearly this is unintended.