Isso é normal. O Word tenta adivinhar qual será seu próximo estilo, marcando suas entradas anteriores. Tente isso para eliminar esse comportamento:
Word 2007-2013
click on the Home tab / Styles group / Styles dialog launcher (the little arrow in the right bottom corner) to launch the Styles Pane. Then > click on the style you using and click on modify. Check the name of the style in the "Style for following Paragraph" input field. If it is something unexpected, change it and save it to normal template.
Word 2016
click on the Home tab / Styles group / Styles dialog launcher (the little arrow in the right bottom corner) to launch the Styles Pane. Then > click on the style you using and click on modify. Go to format and select Fonts. In the input Field select +Body and check if there is something unusual.