**I'm guessing a chunk save failure, perhaps caused by 1.14**

I don't think the house was struck by lightning.  For that much damage to have been done it would need to burn for quite some time.  Fire is slow to destroy blocks, and doesn't spread quickly.  Unless you were nearby the chunk wouldn't have been loaded and the fire would not spread.  If you went straight to the island you were exploring and straight back then you definitely would have seen any fire that destroyed that many blocks.

Your house probably isn't in a spawn chunk as it wasn't loaded for very long once the floor was gone.  We can tell because none of the dirt below the floor has turned to grass yet ([grass spreads][1] anywhere in a 3x3x5 area).

You mentioned creepers and endermen.  Creepers are powerful and would destroy the dirt under the planks (not to mention there would be nothing to aggro them if you're gone).  Endermen [cannot pick up][2] any wooden block.

I don't believe this is a whole-chunk error.  It appears part of the roof is overhanging areas of the floor that were removed and vice versa.  Also, the affected areas didn't revert to their seed-generation state.  If so your floor would be replaced with grass.

To me, the best explanation is that either the chunk failed to fully save or had errors when loading.  Given the very specific damage, my best guess is a save failure.  Due to that failure, affected areas reverted to a semi-random prior state when you were building the house.  This is the game behavior being exploited in the well-known [chunk-overflow dupe][3].

There were bugs in 1.14 that caused crashes and data loss, such as [MC-147754][4].  There were also numerous chunk issues with 1.14.  If you updated around that time then maybe you were affected by one.


Note:  Intended to comment on Quijibo's answer but don't have the rep yet, so taking a stab at an answer for you.


  [1]: https://minecraft.gamepedia.com/Grass_Block
  [2]: https://gaming.stackexchange.com/questions/185341/what-blocks-can-endermen-pickup
  [3]: https://www.youtube.com/watch?v=sd8SM8YWP9U
  [4]: https://bugs.mojang.com/browse/MC-147754