By editing terrain_texture.json, the bug is easily fixed. This bug is not a bug and now invalid.

Update: The game needs a deeper fix than editing terrain_texture.json. Even creators from the Marketplace want a proper fix. See https://twitter.com/Sphax84/status/1168847920654901248

Installing resource packs higher than the 64x resolution (64x, 128x, etc.) will result in the blocks becoming very blurry in the inventory on the Windows 10 Edition. This can make it very hard to distinguish different types of similar blocks (see attached screenshots).

This only seems to happen with blocks. The items are easily recognizable and not blurry.

This only happens on the Windows 10 Edition, not the pocket edition.

It only happens in the Inventory and Hotbar. Holding the block will show the normal block.

To reproduce this issue:
1. Download a 64x or 128x resource pack (like this one: http://www.mediafire.com/file/vruehu6pucwupm8/faithful64rev5hotfix.mcpack)
2. Install the mcpack file and load the Windows 10 Edition of the game.
3. Create or load a world and observe the blurry blocks.

Many people have told me about the issue, and it can be very annoying trying to find the correct block.

Screenshots:
block_bug.png is on the Windows 10 Edition with a 64x pack
block_no_bug.png is on a different 64x pack on Pocket Edition (Android)