Jumping in my pygame program not working – jumps to the top of screen and goes below platform

I have had this issue for a while now and I have looked for the solution and have tried many things but nothing I do seems to work. What is happening is when I press my jump key the sprite jumps up very high the first time and then every time after that it jumps properly but goes below the ground. I will try to attach a video somehow to show what I mean.

Here is the function that has the jumping within it:

def level1():
    global running, jumping1, neg1
    global x1, y1 , x2, y2, xChange1, yChange1, xChange2, yChange2, jump1, jump2, jumping1, jumping2, isJump

    
    while running:
        clock.tick(FPS)
        
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                running = False
            if event.type == pygame.KEYDOWN:
                if event.key == pygame.K_LEFT:
                    xChange1 = -5
                if event.key == pygame.K_RIGHT:
                    xChange1 = 5
                if event.key == pygame.K_UP:
                    if not jumping1:    
                        jumping1 = True
            if event.type == pygame.KEYUP:
                if event.key == pygame.K_LEFT or event.key == pygame.K_RIGHT:
                    xChange1 = 0


            if event.type == pygame.KEYDOWN:
                if event.key == pygame.K_a:
                    xChange2 = -5
                elif event.key == pygame.K_d:
                    xChange2 = 5
                elif event.key == pygame.K_w:
                    yChange2 = -5
                elif event.key == pygame.K_s:
                    yChange2 = 5
            if event.type == pygame.KEYUP:
                if event.key == pygame.K_a or event.key == pygame.K_d:
                    xChange2 = 0
                elif event.key == pygame.K_s or event.key == pygame.K_w:
                    yChange2 = 0           

        if jumping1:
            if jump1 >= -25:
                neg1 = 1
                if jump1 <0:
                    neg1 = -1
                y1 -= (jump1 **2) * 0.5 * neg1
                jump1 -= 1
            else:
                jump1 = 10
                jumping1 = False
 
        x1 += xChange1
        x2 += xChange2
        y1 += yChange1
        y2 += yChange2

        gravity1(y1)
        
        screen.blit(game_image,(0,0))
        character1(x1,y1)
        character2(x2,y2)
        gameFloor(fx,fy)

        if x1 < 0:
            x1 = 0
        if x1 > WIDTH - Char1WIDTH:
            x1 = WIDTH - Char1WIDTH

        if y1 < 0:
            y1 = 0
        if y1 > HEIGHT - Char1HEIGHT - 50:
            y1 = HEIGHT - Char1HEIGHT - 50

        if x2 < 0:
            x2 = 0
        if x2 > WIDTH - Char2WIDTH:
            x2 = WIDTH - Char2WIDTH

        if y2 < 0:
            y2 = 0
        if y2 > HEIGHT - Char2HEIGHT-50:
            y2 = HEIGHT - Char2HEIGHT -50


            
        pygame.display.flip()

Here is a link including my whole code: https://trinket.io/python/8d14105b3d

Here is a link to a video showing my program: https://kapwi.ng/w/50KogQ7A