In anticipation of the release of .net core 3.0, I want to pay attention to the error when converting files using the Aspose.Words library. It appears only if PublishTrimmed is specified in the publication settings.
We have logged a ticket for support of .NET Core 3.0 in Aspose.Words for .NET as WORDSNET-19152. You will be notified via this forum thread once there is an update available on it.
The reason of your problem is that minificator does not see code in virtualized methods and as a result it does not include libraries required by this code into the final build. To work the problem around the you can include this option into the project: