mergeScan drops accumulator with async merging source

Bug Report

Current Behavior

sync

Rx.range(1, 3).pipe(

    Rx.mergeScan((acc, n) =>

        Rx.of([ ...acc, n ])

    , [ 0 ]),

).subscribe(console.log);

// [ 0, 1 ]
// [ 0, 1, 2 ]
// [ 0, 1, 2, 3 ]

async

Rx.range(1, 3).pipe(

    Rx.mergeScan((acc, n) =>

        Rx.of([ ...acc, n ]).pipe(
            Rx.delay(5),
        )

    , [ 0 ]),

).subscribe(console.log);

// [ 0, 1 ]
// [ 0, 2 ]
// [ 0, 3 ]

Expected behavior

Async accumulating as well as synced one.

Reproduction

stackblitz

Environment

• Runtime: Node v12 v14 v16
• RxJS version: v7.3.0

Possible Solution

Rx.range(1, 3).pipe(

    Rx.mergeScan((acc, n) =>

        Rx.of([ ...acc, n ]).pipe(
            Rx.delay(5),
        )

    , [ 0 ], 1),  // set concurrent to ONE works but loosing parallelism

).subscribe(console.log);

Additional context/Screenshots

#2737

Run the linked StackBlitz reproduction and compare the synchronous and delayed mergeScan examples. Investigate the mergeScan behavior described in issue #2737, then verify that asynchronous accumulation retains prior values while preserving concurrency greater than one.