First of all, I would like to share that ConcatenateForms method is now obsolete, so it is better that you use the Concatenate method for future. However, if you’re having any problems using this method, we’ll try to provide you a fix for that.
Secondly, please try to download the latest version (4.9.0) and test it at your end with Concatenate method. If it still doesn’t resolve your issue then please share a small sample application along with the input PDF files which can help us reproduce the issue at our end.
We’re sorry for the inconvenience and looking forward to help you out.