Issues with Mail.app AppleScript scripts

I'm trying to get a simple AppleScript to run from a rule in Mail.app under Catalina. The script is very basic:

use AppleScript version "2.4" -- Yosemite (10.10) or later
use scripting additions

using terms from application "Mail"
    on perform mail action with messages messageList in mailboxes mbox for rule aRule

        repeat with theMessage in theMessages
            set thisSender to (sender of theMessage as string)
            my WriteLog("Recieved email from " & thisSender)
        end repeat

    end perform mail action with messages
end using terms from

-- the code, from here on, has been tested and runs from Script Debugger or Script Editor
on write_to_file(this_data, target_file, append_data) -- (string, file path as string, boolean)
    try
        set the target_file to the target_file as text
        set the open_target_file to ¬
            open for access file target_file with write permission
        if append_data is false then ¬
            set eof of the open_target_file to 0
        write this_data to the open_target_file starting at eof as «class utf8»
        close access the open_target_file
        return true
    on error
        try
            close access file target_file
        end try
        return false
    end try
end write_to_file

on WriteLog(the_text)
    set this_file to (((path to desktop folder) as text) & "log.txt")
    my write_to_file(the_text, this_file, true)
end WriteLog

I have added the following rule to Mail.app:

I have also enabled full disk access for Mail.app:

The rule runs – the messages are moved to the filing folder – but the log file is not updated.

What else should I be looking at, to try to solve this issue?

Update

I noticed a small gear icon appearing in the menu bar when the script was run. Through some manoeuvring I managed to right-click the icon, and capture the resulting menu. I suspect this is some automation indicator ( possibly CoreCervices/ScriptMonitor ?)

I therefore tried enabling CoreCervices/ScriptMonitor in the FullDisk Access pane of the Security & Privacy's 'Privacy' tab… no joy.

Apart from the tccutil reset, does anyone know if there is a way to trigger a request from Mail.app for AppleEvent access of say the Finder, so that an option appears in the 'Automation' tab of Security & Privacy?

Update:

Here's the full script, including the addEmailToDB() function. It works fine as a standalone, manually run, but doesn't respond when run from Mail.app. The Allow Apple events... extended privilege is enabled for Admin, and the FM file is set to open as Admin, with no password, and is already open before the script runs.

use AppleScript version "2.4" -- Yosemite (10.10) or later
use scripting additions

using terms from application "Mail"
    on perform mail action with messages theMessages in mailboxes mbox for rule aRule

        repeat with theMessage in theMessages
            set thisSender to (sender of theMessage as string)
            set thisSubject to (subject of theMessage as string)
            set thisContent to (content of theMessage as string)
            my WriteLog("Recieved email from " & thisSender & ", Subject: " & thisSubject & return)
            my addEmailToDB("mails", "mails", thisSender, "", "", "", thisSubject, thisContent)
        end repeat

    end perform mail action with messages
end using terms from

-- the code, from here on, has been tested and runs from Script Debugger or Script Editor
on write_to_file(this_data, target_file, append_data) -- (string, file path as string, boolean)
    try
        set the target_file to the target_file as text
        set the open_target_file to ¬
            open for access file target_file with write permission
        if append_data is false then ¬
            set eof of the open_target_file to 0
        write this_data to the open_target_file starting at eof as «class utf8»
        close access the open_target_file
        return true
    on error
        try
            close access file target_file
        end try
        return false
    end try
end write_to_file

on WriteLog(the_text)
    set this_file to (((path to desktop folder) as text) & "log.txt")
    my write_to_file(the_text, this_file, true)
end WriteLog

on addEmailToDB(theDB, theTable, sender, recipient, cc, bcc, subject, body)
    try
        tell application "FileMaker Pro 18 Advanced"
            activate
            show database theDB
            tell table theTable of database theDB
                set newRec to create new record
                show newRec
                tell newRec
                    set cell "from" to sender
                    set cell "to" to recipient
                    set cell "cc" to cc
                    set cell "bcc" to bcc
                    set cell "subject" to subject
                    set cell "body" to body
                end tell
            end tell
        end tell
        return true
    on error
        return false
    end try
end addEmailToDB