I have a Word template used for mail merging via the Java version of Aspose.Words that used to work with version 16.7. Earlier this year we upgraded to version 18.1 and the same template now generates bookmark not defined errors. I noticed that the release notes for Aspose.Words 18.8 have a reference to a fix WORDSNET-16904 which sounds exactly like the problem I am having. However after downloading version 18.8 I am still having the same issue. If I remove the call to updateFields after the mail merge then the generated document does not contain the error message.
Can you confirm that the fix for WORDSNET-16904 is included in the Java release version 18.8. If so it looks like there is another similar issue following a mail merge.