I have looked through several Expert Advisors on the forum with the word “multicurrency” in their names.
No one of them could be optimized for several currencies in one optimization run.
I had to invent …

extern   int      idxSymbols;
         string   arrSymbols[3];
         string   currSymbol;

OnInit ()
{
...
   arrSymbols[0]="AUDNZD";
   arrSymbols[1]="EURUSD";
   arrSymbols[2]="GBPUSD";

// Вариант 1 - классический : текущий символ = символу из настроек тестера
   currSymbol=Symbol();         
// Вариант 2 - оптимизация : текущий символ = элементу массива, индекс которого перебирается как input параметр при оптимизации
   currSymbol=arrSymbols[idxSymbols]; //
...
}

Simulation: Opening prices only

Testing each pair separately by the first variant, net profit:
AUDNZD – 4180,96
EURUSD – 8674,95
GBPUSD – 1686,79

Optimization by the second variant : The Parameters tab goes through idxSymbols from 0 to 2 inclusive, with the GBPUSD symbol in the tester settings

That’s right. But ….
Changed the symbol for EURUSD in the tester settings: I changed profit on GBPUSD [Pass 2]

Changed the symbol for AUDNZD in the tester settings: I got wrong profits on EURUSD [pass 1] and GBPUSD [pass 2]

What is it ? Help … please … very please 🙁

UPD Cleaning the cache didn’t help