Mud Bricks Slabs and Stairs do not tile texture correctly with adjacent Mud Bricks blocks.

Most full blocks use a "minecraft:block/cube_all" template model, but the Mud Bricks block model "minecraft:block/mud_bricks_north_west_mirrored" uses a different template "minecraft:block/cube_north_west_mirrored_all" that specifically mirrors the north and west facing sides of the cube, presumably to prevent a texture tiling error, or perhaps look better around corners, I’m not entirely sure why. Mud Bricks are the only block in the game to use this mirrored cube template.

However, the Mud Brick Slab, when in its "type=double" state, uses a different model "minecraft:block/mud_bricks" that just uses the normal "minecraft:block/cube_all" template, which causes a texture tiling error when adjacent to normal Mud Bricks blocks. Additionally, the Mud Bricks item uses the non-mirrored model, and Mud Bricks Stairs are also affected by the issue as they use the standard stairs templates.

Note how in the attached screenshots, when viewing the north & west faces of Mud Bricks slabs or stairs, the texture does not tile properly with the adjacent Mud Bricks blocks, but when viewing the south & east faces the texture does tile properly (as the Mud Bricks blocks do not mirror this side).

A possible fix is to remove the non-mirrored version of the Mud Bricks block model ("minecraft:block/mud_bricks"), and have the double slab & item use the same model ("minecraft:block/mud_bricks_north_west_mirrored") as the block.
Alternatively, the mirroring could be entirely removed from the Mud Bricks block model, making it match all other bricks (and most blocks in general) in the game, which may be an easier fix (not requiring tons of mirrored forms of stairs).