Check out the documentation for ACF image fields: https://www.advancedcustomfields.com/resources/image/
When you use get_field on an image field, it looks like it returns the ID for the image, not a URL. This might have been different in a previous version of ACF.
I think something like this might work:
$image = get_field('user_avatar', $user->ID');
$size="full"; // (thumbnail, medium, large, full or custom size)
if( $image ) {
echo wp_get_attachment_image( $image, $size );
}