This is my opinion, so check if it makes sense to you.
Personally I think what makes sense is that if you set maxLines=null it logically means it is unbounded, reason why the textField should inherit it's parents constraints instead of starting with one line and growing to it.
If the current behaviour is required by some people, then I suggest a flag be added to control whether it should minimize or maximize it's footprint, similarly to the column with it's MainAxisSize.min and MainAxisSize.max
This is what I propose so that the TextField's syntax makes a little more sense, at least to me.