1. With a FortiAP advanced management license, you can enable the following advanced settings:
2. As authentication is working and only accounting is not, you could try below:
a. Enable the following logs on the FAP SSH in question to see if we can get some hints:
# cfg -a ADMIN_TIMEOUT=0
# cfg -c
# cw_debug app hostapd 15
# cw_debug on
Turn off logs using command "cw_debug off" and setting "cw_debug app hostapd 0"
b. With the RADIUS accounting server method, the Accounting Interim Interval parameter becomes available. The AP will send an Interim Update Accounting-Request to update the RADIUS accounting server with time and bandwidth usage. The default value is set to 600 seconds (or 10 minutes).
You may try collecting AP uplink port-mirror captures if the hostapd logs do not help: