Thanks for your patience. It is to inform you that our product team has completed the work on this issue (WORDSNET-12686) and has come to a conclusion that this issue and the undesired behavior you're observing is actually not a bug in Aspose.Words. So, we have closed this issue as 'Not a Bug'. We are quoting developer's comments here for your reference.
InsertHtml is instructed to merge target formatting with formatting specified in HTML fragments. However, formatting of the first merge field in the target document differs from formatting of other merge fields and, as a result, formatting of text inserted into the first merge field differs from formatting of text inserted to other merge fields.
Please remove direct font formatting of the first merge field to get the required output.
Document doc = new Document(MyDir + "headingsBroken.docx");
var builder = new DocumentBuilder(doc);
1 New Section
Title test", true);
Fixed Bid", true);
Unit of Measure", true);
doc.Save(MyDir + "Out.docx", SaveFormat.Docx);