I also have hit this issue. Numbered list styles are not being imported correctly when using the KeepSourceFormatting import mode, with lists incorrectly continued. This scenario occurs when importing documents into a master document with DocumentBuilder.
I have been developing an proof-of-concept using Aspose.Words .NET 18.10 and this issue has had me scratching my head for a couple of days. Rolling the package back to 17.6 fixes the problem. If it weren’t for this post I would have never have known.