That is correct, it seems the auto-fill is only correct if all the trades are CDN. I do not understand why CRA still permits both CAD and USD versions to be mixed together. It is also not clear where the problem is, maybe the CDN/USD information is missing when the broker sends it to CRA.
Anyway, the USD do need to be converted. The most accurate way is to make an Excel sheet and use the exchange rate for each trade and then enter the values into TurboTax.
On the TaxTips.ca site (https://www.taxtips.ca/filing/reporting-foreign-transactions.htm) it states we cannot use the average annual exchange rate (I haven't looked on the CRA site exactly where it states this), but we truly have no choice. If the shares were bought at different times and even in different years and then sold in 2021, the average cost of the shares includes a mix of more than one exchange rate. If I transferred $500 and then $1,000 CDN into a USD trading account and then bought US shares in 2021 - do I use 33% of the exchange rate of the $500 xfr and 66% of the rate for the $1,000 xfr for the stock purchase, or the rate when I bought the shares because that's what their CDN value would have been on that day even though I used US dollars from a different rate? That becomes so complicated, it's guaranteed to be inaccurate.
So I use the annual CAD-USD exchange rate, found on https://www.bankofcanada.ca/rates/exchange/annual-average-exchange-rates/
CRA says this rate should only be used for "income throughout the year" but in a way, my trades are throughout the year so I apply the same logic.
(That's my view about the only way this can be done reasonable and accurately, I'm not a certified financial person)