I also found the same issue as you where the historical ticks are rounded to the 1 second, and after much searching couldn't find any way to get higher resolution from ibkr.
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I wasn't sure that I could trust that the order of the ticks would be the same order that they would have come in via the live tickbytick method.. so I just went and bought historical ticks from databento for the backtest time range.
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It's a bit of a pain to use another data source, map it all, etc.. but at least now I'm confident that the data is accurate. Good luck.
wordd