Certain secrets (character combinations) are not stored correctly by the ISAPI agent.
'Welcome123' gets stored as 'Welco??????3' and 'websiteqpm' gets stored as 'websiteqp'.
However, 'TestISAPI123' gets stored correctly as 'TestISAPI123'.
1. Use secret TestISAPI123 to confirm the configuration is correct then update to a different secret.
2. Enable debug using the steps provided in Debugging the Defender ISAPI Agent to verify the secrets are stored correctly.
Waiting for a fix in a future release of Defender.