https://drafts.csswg.org/css-grid-1/

Distribute space to base sizes up to growth limits: Find the item-incurred increase for each spanned track with an affected size by: distributing the space equally among such tracks, freezing a track’s item-incurred increase as its affected size + item-incurred increase reaches its growth limit (and continuing to grow the unfrozen tracks as needed). If a track was marked as infinitely growable for this phase, treat its growth limit as infinite for this calculation (and then unmark it).
Note: If the affected size was a growth limit, each item-incurred increase will be zero.

So this entire step effectively applies only to base sizes, not to growth limits, since all it does to growth limits is set them to zero. It would be nice if the editorial structure was clearer about this up front.