max unevaled in plot??

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Dear all,

Here is a puzzling code:

var('a')
show(plot(max([1-a, a]), (a,0,1)))
show(plot(max([a, 1-a]), (a,0,1)))

Only the first term of the list is used in this maximum!???

Any hint is welcome, I can sure plot a polygon made of sample points, this works, but I would rather understand why the above simple piece fails to behave as I would expect.

Many thanks in advance! Olivier