It's known that some Chinese mobile apps employ this trick to keep the app alive in the background, the rumor is that this way the 'active user' KPI can be better met.

edit: quantity qualifier

Why would they manipulate the numbers that way if it’s self-reported anyways? If you want to fake the numbers, couldn’t you just change the number and be done?

The people fudging the metric (developers) may not be the same people that are relying on the metrics (managers). Perverse incentives at play could easily motivate this kind of shenanigan.

It's probably quite difficult to manipulate per-user metrics after the data is collected.

If it's known, are you suggesting that Apple and Google are complacent in allowing these type of apps in their ecosystem?

I wouldn't think of giant organizations like Apple and Google as mere individuals that can exercise human emotions such as complacency.