Cannot move while sneaking when player's generic.step_height ≥(Collidable block's hitbox total height under the player)+1.5

Thank you for your report!
We're actually already tracking this issue in MC-267381, so this ticket is being resolved and linked as a duplicate.

If you would like to add a vote or any additional information to the main ticket it would be appreciated.

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This is because player avoids to fall from block edge, but the checking method is wrong.

@Override
protected Vec3 maybeBackOffFromEdge(Vec3 var0, MoverType var1) {
    if (!this.abilities.flying && var0.y <= 0.0 && (var1 == MoverType.SELF || var1 == MoverType.PLAYER) && this.isStayingOnGroundSurface() && this.isAboveGround()) {
        double var2 = var0.x;
        double var3 = var0.z;
        double var4 = 0.05;
        while (var2 != 0.0 && this.level().noCollision(this, this.getBoundingBox().move(var2, -this.maxUpStep(), 0.0))) {
            if (var2 < 0.05 && var2 >= -0.05) {
                var2 = 0.0;
                continue;
            }
            if (var2 > 0.0) {
                var2 -= 0.05;
                continue;
            }
            var2 += 0.05;
        }
        while (var3 != 0.0 && this.level().noCollision(this, this.getBoundingBox().move(0.0, -this.maxUpStep(), var3))) {
            if (var3 < 0.05 && var3 >= -0.05) {
                var3 = 0.0;
                continue;
            }
            if (var3 > 0.0) {
                var3 -= 0.05;
                continue;
            }
            var3 += 0.05;
        }
        while (var2 != 0.0 && var3 != 0.0 && this.level().noCollision(this, this.getBoundingBox().move(var2, -this.maxUpStep(), var3))) {
            var2 = var2 < 0.05 && var2 >= -0.05 ? 0.0 : (var2 > 0.0 ? (var2 -= 0.05) : (var2 += 0.05));
            if (var3 < 0.05 && var3 >= -0.05) {
                var3 = 0.0;
                continue;
            }
            if (var3 > 0.0) {
                var3 -= 0.05;
                continue;
            }
            var3 += 0.05;
        }
        var0 = new Vec3(var2, var0.y, var3);
    }
    return var0;
}

It should search the collision from player's feet to its max step height, not just move the collision box to the max step height and check whether it collides.

Please provide your findings on MC-267381 instead of here.