Not sure if this is a known issue, apologies if it is.
On the default “Your Outputs” pages (mine - https://pvoutput.org/list.jsp?userid=7897) the overall balance for the previous month/28 days doesn’t seem to account for off-peak import prices. On my current month I am £10.86 down as I have imported 345kWh + daily charges. A good half of that is off peak and is correct if I look at each day individually - most days are in profit over the last month.