According to https://github.com/AltBeacon/android-beacon-library/issues/418, As of Android N, DP4:

    We’ve changed the BLE Scanning behavior starting in DP4. We’ll prevent
    applications from starting and stopping scans more than 5 times in 30 
    seconds. For long running scans, we’ll convert them into opportunistic scans.

And if you look into https://android.googlesource.com/platform/packages/apps/Bluetooth/+/master/src/com/android/bluetooth/gatt/AppScanStats.java#59, you'll see two constants:

    static final int NUM_SCAN_DURATIONS_KEPT = 5;

    // This constant defines the time window an app can scan multiple times.
    // Any single app can scan up to |NUM_SCAN_DURATIONS_KEPT| times during
    // this window. Once they reach this limit, they must wait until their
    // earliest recorded scan exits this window.
    static final long EXCESSIVE_SCANNING_PERIOD_MS = 30 * 1000;

    ...


    synchronized boolean isScanningTooFrequently() {
        if (lastScans.size() < NUM_SCAN_DURATIONS_KEPT) {
            return false;
        }
        return (System.currentTimeMillis() - lastScans.get(0).timestamp) <
            EXCESSIVE_SCANNING_PERIOD_MS;
    }

And look at https://android.googlesource.com/platform/packages/apps/Bluetooth/+/master/src/com/android/bluetooth/gatt/GattService.java#1332,


        if (app != null) {
            if (app.isScanningTooFrequently() &&
                checkCallingOrSelfPermission(BLUETOOTH_PRIVILEGED) != PERMISSION_GRANTED) {
                Log.e(TAG, "App '" + app.appName + "' is scanning too frequently");
                return;
            }
            scanClient.stats = app;
            app.recordScanStart(settings);
        }
        mScanManager.startScan(scanClient);

Confirming the findings above. I could not find any way to make Chromium detect the condition where scanning is skipped. 
This is what I see in the log:

BtGatt.GattService: onScanFilterParamsConfigured() - clientIf=5, status=0, action=1, availableSpace=16
BtGatt.GattService: registerClient() - UUID=6c63c486-b863-46f6-9b0e-a1818bb0d4dc
BtGatt.GattService: onClientRegistered() - UUID=6c63c486-b863-46f6-9b0e-a1818bb0d4dc, clientIf=5
BtGatt.GattService: start scan with filters
BtGatt.GattService: App 'org.chromium.chrome' is scanning too frequently
BtGatt.GattService: stopScan() - queue size =0
BtGatt.GattService: unregisterClient() - clientIf=5
BtGatt.GattService: registerClient() - UUID=c0c033bf-b66b-4184-abe0-ae1138a2cbba
BtGatt.GattService: onClientRegistered() - UUID=c0c033bf-b66b-4184-abe0-ae1138a2cbba, clientIf=5
BtGatt.GattService: start scan with filters
BtGatt.GattService: App 'org.chromium.chrome' is scanning too frequently

Shall we ask Android folks to at least raise an error when starting scan instead of failing silently?
Or should we monitor ourselves time between scan and scan lengths?

I would have preferred Android to raise an error.

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available. If you change it back, also remove the "Hotlist-Recharge-Cold" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot