MC-217906 – Large amounts of axolotls cause performance issues.
MC-229365 – Large amounts of goats cause performance issues.
From the development versions
MC-236632 – Banners without patterns do not render properly.
MC-236663 – Command blocks do not display command in their UI.
MC-236736 – No relief on treasure map or explorer maps.
MC-236896 – Strange mini broken aquifer walls generating commonly on the surface.
MC-237490 – The bottom parts of aquifers aren't always surrounded by blocks which can occasionally cause them to spill into caves in ways that appear bizarre.
From the previous development version
MC-237925 – Opening world from 1.12.2 (or older) in latest snapshot regenerates random chunks.
MC-237933 – Sweet Berries generate much too often.
MC-237950 – Cave vines can generate without tip because prioritize_tip option is negated.
MC-175929 – Ice spikes get cut off below height limit on AMPLIFIED worlds.
MC-182362 – Score callbacks with a too long name break callback chain.
MC-183184 – Sprinting stops as soon as you touch another block.
MC-186042 – Plains biome is always generated when a swamp borders a desert, snowy tundra or snowy taiga.
MC-188096 – Gravelly mountains+ is no different than gravelly mountains.
MC-190363 – Some biomes generate as "Single biome" worlds in "Floating islands" world type.
MC-193348 – Status effect bars shift the player's inventory in Creative mode.
MC-196723 – If the player gets an effect in Creative mode while their inventory is open and not having an effect before, they won’t see the effect in their inventory until they close and open their inventory.
MC-202376 – Rabbits, instead of spawning on snow blocks, spawn on snow layers.
MC-208601 – Axolotls do not avoid danger when pathfinding.
MC-212113 – Glow lichen can spawn underwater whilst not in a cave.
MC-213779 – FPS dramatically decreases when looking up in world with large height.
MC-214335 – Flower forest is much steeper than regular forest.