The component painting/variant can accept inline definition, like this:

/give @s minecraft:painting[minecraft:painting/variant={asset_id:bust,height:3,width:3}]

This command was parsed successfully, and you can place the right paintings. But the painting entity do not save variant data when the format of painting/variant is defined inline in 25w04a, which means when you exit and re-enter the level the painting will use default variant "alban".
In 25w03a, painting entity can save inline defintion variant.

How to Reproduce:

These steps needs to be done on both 25w03a and 25w04a to compare the differences.

1. Run above command to get a painting.
2. Place it.
3. Exit the world, and re-enter it.
4. Check the painting variant.

Observed Result:

Inline difinition painting variant is saved in 25w03a, do not saved in 25w04a.

Note

Because the changelog doesn't say anything about this feature, I'm not sure which one is intentional and which one is unexpected. But for now, inline definition painting/variant does not make any sense.