User informed us following example:

int nDec = 0;
QLocale::setDefault(QLocale(QLocale::Spanish, QLocale::Spain));
double d=2004828;
qDebug()<< QString("%L1").arg(d, 0, 'f', nDec);

QLocale l;
qDebug()<< l.toString(d, 'f', nDec);

They were getting "2004.828" instead of "2.004.828" in WASM and the user is located in Spain.

Qt Support tested the example outside of Spain and the end results were on both desktop and WASM "2004.828" which is not following the Spanish thousand separation:

https://docs.oracle.com/cd/E19455-01/806-0169/overview-9/index.html