What is the logic behind unrealised FX gain/loss not being included in the TB and why the prior month's gain/loss is reversed out in the following month?

Hello Nancy-barber-etr, thanks for posting on the Community page, So in QuickBooks Online , unrealised gains or losses do not appear in the Trial Balance report because the Trial Balance report is designed to show the balances of accounts as of a specific date, and unrealized gains or losses are not realized until a transaction is completed. This means that the unrealised figure isn't an actual amount yet, but if the transaction(s) were completed (paid) now that is what the exchange gain/loss would be.  If you want to make adjustments on the balances, you would need to revalue the currency. You can also speak to an accountant who can guide you more on this too.