Typescript 为什么第二次订阅会改变我的直播结果

以下代码按预期工作(请参见输出):

import { Observable, Subject } from "rxjs";
import { scan } from "rxjs/operators";
export type ColumnFilter = { field: string, value: string };

let sub$: Subject<ColumnFilter> = new Subject<ColumnFilter>();
let obs$: Observable<ColumnFilter[]> = new Observable<ColumnFilter[]>();

obs$ = sub$.pipe(
    scan((acc: ColumnFilter[], curr) => {
        const index = acc.findIndex(f => f.field === curr.field);
        if (index === -1) {
            acc.push({ field: curr.field, value: curr.value });
            return acc;
        }

        curr.value === '' ?
            acc.splice(index, 1) :
            acc[index] = { field: curr.field, value: curr.value };
        return acc;
    }, [])
);


obs$.subscribe(
    value => console.log(value)
);

sub$.next({ field: 'size', value: 'xl' })
sub$.next({ field: 'color', value: 'black' })
sub$.next({ field: 'color', value: '' })


// output:
//    [ { field: 'size', value: 'xl' } ]
//    [ { field: 'size', value: 'xl' }, { field: 'color', value: 'black' } ]
//    [ { field: 'size', value: 'xl' } ]

现在，如果我向obs$个可观察对象添加第二个订阅，输出将更改:(判断流的最后一个值.它现在包含{ field: 'color', value: '' }个)

import { Observable, Subject } from "rxjs";
import { scan } from "rxjs/operators";
export type ColumnFilter = { field: string, value: string };


let sub$: Subject<ColumnFilter> = new Subject<ColumnFilter>();
let obs$: Observable<ColumnFilter[]> = new Observable<ColumnFilter[]>();

obs$ = sub$.pipe(
    scan((acc: ColumnFilter[], curr) => {
        const index = acc.findIndex(f => f.field === curr.field);
        if (index === -1) {
            acc.push({ field: curr.field, value: curr.value });
            return acc;
        }

        curr.value === '' ?
            acc.splice(index, 1) :
            acc[index] = { field: curr.field, value: curr.value };
        return acc;
    }, [])
);

// vvv this was added
obs$.subscribe()
// ^^^ this was added

obs$.subscribe(
    value => console.log(value)
);

sub$.next({ field: 'size', value: 'xl' })
sub$.next({ field: 'color', value: 'black' })
sub$.next({ field: 'color', value: '' })

// output:
//    [ { field: 'size', value: 'xl' } ]
//    [ { field: 'size', value: 'xl' }, { field: 'color', value: 'black' } ]
//    [ { field: 'size', value: 'xl' }, { field: 'color', value: '' } ]

我不明白为什么第二次订阅会改变流的结果.我只能猜测这与变异累加器有关？因为如果我使用acc.filter而不是acc.splice，它就会像预期的那样工作.

Edit个

增加了一个堆叠闪电战:https://stackblitz.com/edit/rxjs-6-opeartors-uzojgw?file=index.ts