Optimization inconsistency

Hi and thanks for the help

I am relatively new to Forex, but am a software developer, so most EA concepts I grasp. I wrote a simple stochastic EA which I am now back testing. A few points:

• I coded a check into the start() function to ensure the EA only opens/closes trades once per bar. I did this because I read that when a EA runs on a live chart the processing mode is always "every tick", and I needed max one trade opened per bar.

if (Bars == ThisBarTrade ) {
   return(0);
}
   
ThisBarTrade = Bars;

• Consequently I am now backtesting the EA in "Every Tick" mode, to simulate a live environment. Results are as expected: trades are only opened or closed every hour on the hour on a 1H chart. I get a good return from the EA, curve-fitting notwithstanding (I'll address that when I get to the walk-forward optimization).
• However when I run the backtest in "Open prices only" in order to speed up the testing, I get very bad results. In fact, no combination of parameters yields a positive return. I'm not sure if this is relevant, but looking at the Results tab on an individual test (open prices only), trades are now all happening at 1 minute before the hour, i.e, 09:59 instead of at the bar opening time of 10:00.