Summary
iText 5.5.12 for .NET does not appear to validate a commercial license correctly when used with version 3.1.6 of the itext7.licensekey library.
.NET Behaviour
In .NET environments, when iText 5.5.12 is used with license key version 3.1.6, generated documents are still created in AGPL mode with no error or exception. This behavior appears to be .NET-specific, as the same license file loads correctly with iText 5 for Java.
Suggested Solution
With the release of iText 5.5.13, the license key library version requirements changed from version 1.x to 3.x, to match the then current iText 7.1.x release.
Since this issue does not occur with iText 5.5.13, the recommended approach is to upgrade to the latest version of iText 5 for .NET, and and use the latest compatible itext7.licensekey package (https://www.nuget.org/packages/itext7.licensekey). At the time of writing, the latest iText 5 release is 5.5.13.6, and the latest itext7.licensekey version is 3.1.6.
For future versions, you can consult the https://apryse.atlassian.net/wiki/x/XgApGg for up-to-date compatibility information.