https://drafts.csswg.org/css-grid/#min-size-contribution says:

[...] if its specified size ('width' or 'height', whichever matches the relevant axis) is 'auto', [...]

I suspect that the intent is that percentages should behave as auto when the percentage basis is indefinite, so I think it would be clearer to use that phrase (linked to the definition) instead of 'auto'.