Soft delete modifies custom date field in database table

I have a table with a field 'expires_at'. When soft deleting an entry and all it's children, it has it's deleted_at and updated_at field change. But for some reason, it also changes my custom field expires_at to the same value. Is this the expected behaviour, or am I doing something wrong?

Migrations:

Schema::create( 'licensegroups', function ( Blueprint $table ) {
            $table->increments( 'id' );
            $table->string( 'name' );
            $table->timestamp( 'expires_at' );
            $table->timestamps();
            $table->softDeletes();
        } );

Schema::create( 'licenses', function ( Blueprint $table ) {
            $table->increments( 'id' );
            $table->integer( 'licensegroup_id' )->unsigned();;
            $table->integer( 'quantity' )->unsigned();;
            $table->string( 'key' );
            $table->timestamps();
            $table->softDeletes();
    
            $table->foreign('licensegroup_id')->references('id')->on('licensegroups')->onDelete('cascade');
        } );

Models:

class Licensegroup extends Model
{
    use SoftDeletes;
    protected $dates = ['deleted_at'];

    // ...

    public function licenses()
    {
        return $this->hasMany( License::class );
    }
    
    public static function boot()
    {
        parent::boot();
        
        static::deleting(function($licensegroup)
        {
            $licensegroup->licenses()->delete();
        });
    
        static::restoring(function($licensegroup)
        {
            $licensegroup->licenses()->restore();
        });
    }
}

class License extends Model
{
    use SoftDeletes;
    protected $dates = ['deleted_at'];

    // ...
}

I then use $licensegroup->delete(); to delete the parent with it's children.