Gorm SQL becomes strange after using db.Model

GORM Playground Link

https://github.com/go-gorm/playground/pull/632

Description

Group conditions is not working as expected after calling db.Model

sql1 := DB.ToSQL(func(tx *gorm.DB) *gorm.DB {
    return tx.Where(
        tx.Where("pizza = ?", "pepperoni").Where(tx.Where("size = ?", "small").Or("size = ?", "medium")),
    ).Or(
        tx.Where("pizza = ?", "hawaiian").Where("size = ?", "xlarge"),
    ).Find(&User{})
})

sql2 := DB.ToSQL(func(tx *gorm.DB) *gorm.DB {
+   tx = tx.Model(&User{}) // use db.Model!!!
    return tx.Where(
        tx.Where("pizza = ?", "pepperoni").Where(tx.Where("size = ?", "small").Or("size = ?", "medium")),
    ).Or(
        tx.Where("pizza = ?", "hawaiian").Where("size = ?", "xlarge"),
    ).Find(&User{})
})

These two sqls are expected to be the same. Actually sql2 is the following strange value.

SELECT * FROM `users` WHERE ((pizza = "pepperoni" AND size = "small" OR size = "medium") AND pizza = "hawaiian" AND size = "xlarge") AND `users`.`deleted_at` IS NULL

Why is it like this?