Certain blocks that existed before the 1.2.13 update won't stack with newly mined blocks.

So far I've only seen this bug occur with Mushroom Blocks and all the variants of leaves. Since the 1.2.13 update, such blocks that were stored before the 1.2.13 update will no longer stack with newly mined blocks. I assume that this is due to the removal of the block ID limit. In the video linked, you will see the bug demonstrated on our survival realm, which was up long before the update. https://youtu.be/m-RGd74pKRY
I don't know what other blocks are affected by this, as can be seen, dirt blocks are not.