Radius Accounting atributes with wrong values

Hello,

We have a Fortigate 60E-POE with Forti OS 6.0.5 with an external captive portal and external RADIUS server.

Capturing the RADIUS packets, we can see that the Acct-Output-Octets and Acct-Input-Octets are wrong. For example, 

Acct-Output-Octets=2909370

Acct-Input-Octets=47475008

which means the upload (Acct-Input-Octets) is bigger than the download (Acct-Output-Octets).

Free Radius info about: https://freeradius.org/rfc/rfc2866.html#Acct-Input-Octets

Do you have some information about that issue?

Thanks!

Best regards,

Cristiano