Does surfboard flex occur under the rider's front foot?

Not usually. The reason for this is that water pressure is greatest right under the front foot, and rapidly reduces as it goes aft towards the tail. Water pressure under the front foot is many times greater than at the tail. The high pressure area is like the support used during a land test. Place that support under the front foot and apply as much force as you like and there won't be any flex.

To make the board flex pressure needs to be applied near the fore and aft middle of the wetted surface area, and the tail needs to have a high pressure zone... this is like pushing down on the middle of a board which is supported at the ends.

There are two problems with this:

One is that most boards have no high pressure area at the tail... the tail has very low pressure. With only one high pressure area the situation is like a land test with the board supported at only one end... pushing down in the ;middle' will only depress the tail without any flex. Canted fins or horizontal wings can provide higher pressure in the tail but typical cant is very small and the canted fins are placed fairly far forward which shortens the lever arm and drastically reduces the chance of flex.

The second is that when pressure is placed in the middle of the wetted surface area this moves the high pressure area aft as the waterline length decreases ( because the board raises its nose.)

Another issue is that even assuming that there is a high pressure area at the tail, if the lever arm ( length of board behind the rider's foot) is short then the board won't flex significantly. The amount of flex is greatly increased when there's a long lever arm, for any given force. This also means that a riding position which is right at the tail when turning ( for example on a mal) gives a very short lever arm.

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Long tails are ideal for surffboard flex.

Here's Savitsky's drawing showing typical planing hull for and aft pressure distribution ( the curved line above the hull). As can be seen the high pressure zone is where the rider's front foot is usually. Moving the weight back moves the high pressure zone back as well.