I made a 24x grass top texture for my w.i.p. resource pack. When loading it, I found this message in the game output: (note: mipmap was set to 4)

[13:43:30] [Client thread/WARN]: Texture minecraft:textures/blocks/grass_top.png with size 24x24 limits mip level from 4 to 3

Ingame the texture looks fine.

I don't know why MC decreases the mipmap when using a 24x resourcepack. I haven't tested if the bug occurs when using other resolutions.