Why the random sprite packer sizes?

My sprites are all labelled with the packing tag UI. Unity’s packer has decided to pack them into four textures:

UI (Group 0) - 1024x512
UI (Group 1) - 1024x512
UI (Group 2) - 512x256 (mostly empty space)
UI (Group 3) - 32x32 (single white square!)

Why is this and how do I fix it so a single 2k texture is used? They’d probably even fit a 1k texture if packed efficiently! Target platform is Android if that makes a difference.

Further reserach found this:

If the sprites aren’t of exactly the same type, they aren’t packed into the same texture, which makes sense. For opaque sprites, automatic TrueColour bundles them into 24 bit RGB so they aren’t grouped with RGBA sprites. These can be forced to RGBA using Advanced mode.