16w02a: You can no longer place end crystals inside blocks. However, it is still possible for blocks (obsidian landing platform, obsidian towers, lit end portal, unlit end portal) to be generated in locations where the user has previously placed an end crystal. The end crystal will then replace these blocks with fire. It should instead only replace air and fire with fire.

One such example is demonstrated in the two attached screenshots. In before.png, a piece of bedrock was removed from the unlit end portal. Note that this is possible in survival and that it also is possible for users in older (pre-1.9) worlds to place an end crystal at 0,0 where the end portal will be moved to after summoning another dragon. In after.png, the ender dragon was killed and a lit portal was placed over the unlit portal. The end crystal placed fire inside itself, removing one block of bedrock from the newly placed bedrock frame. Note that without the end crystal the bedrock frame would have been repaired.

Old description:

End crystals keep placing fire at their location even if they are inside a block such as bedrock or end portals. This causes issues like MC-92352 when the game places blocks inside end crystals, especially blocks that are supposed to be unbreakable such as bedrock or end portals. Other affected structures include the obsidian pillars.

Code analysis by marcono1234 can be found in this comment.