Invicible "Lava Source Blocks", These cannot be removed; placing blocks or using an empty bucket won't remove them. This seems pretty similar to MC-12363, it might be related to a bucket desync but it doesn't work in the exact same way.

Attatchment 1: (Cobbleston is placed)

Attachment 2: (Cobblestone Removed)

Attatchment 3: (Flowing Lava source block appears from where the lava was removed)

These lava blocks cannot be removed via bucket. After a few minutes of testing I concluded that the "Lava Source Block" might have turned invisible, using a bucket on a different block made the lava disappear.

Note: This is pretty similar to MC-12363 but it highlights a different issue in the rendering engine rather than the internal server desyncing.  It also is similar to MC-122957 but the requirements for the bug to occur are not the same as the ones depicted in that bug report.