For some reason the billing module is directing my expenses to a tax payable account and not to accounts payable. But this is not happening for all the transactions.

Hello there, alexial.

When you use the Expense option, the funds will directly post to the bank account you use in tracking your tax payments. You can try performing a few browser troubleshooting steps. There are times that the regular browser stores cache file that can cause unusual behavior within the system. 

Try using a private browser and check the Accounts Payable ledger report you pulled up. If the vendor shows the correct currency, you can switch back to the main browser and clear the cache. You'll also want to use other supported browsers to isolate these kinds of issues. 

Let me know how this works. Take care!