Simply, I was switching gamemodes in my new 1.15 world. I'd already collected items, and at some point, I looked down to find my textures were completely warped akin to something from the far lands, after I had shoved everything from Survival into chests. It didn't only affect 2D textures like the inventory item textures, but the block textures as well.

Not pictured is that when going into the Creative index of all items, almost all of the textures were warped in a similar fashion. Diagonal to the top left, almost all the pixels.

The only unaffected items were those with enchantment glows on them, such as Golden Apples and certain potions. Un-enchanted potions (i.e awkward potion) were still warped.

However, when placed, blocks looked completely normal.

Along with that, I noticed suddenly a patch of dirt I'd been placing (during my original Survival Mode antics in the same session) was a completely incorrect tone for the biome I was in. (seen in attached)

I only mention this because it may be one bug causing all of this at the same time.