From https://drafts.csswg.org/css-grid/#algo-init,

For each track, if the track’s min track sizing function is:

• [...]
• A flexible sizing function
  Use an initial base size of zero.

But from https://drafts.csswg.org/css-grid/#min-track-sizing-function,

min track sizing function
If the track was sized with a minmax() function, this is the first argument to that function. If the track was sized with a <flex> value or fit-content() function, auto. Otherwise, the track’s sizing function.

And from https://drafts.csswg.org/css-grid/#valdef-grid-template-columns-minmax

As a maximum, a <flex> value sets the track’s flex factor; it is invalid as a minimum.

So the min track sizing function can't be flexible.