Why does Trebuchet MS font keep sneaking back to Times Roman?

I am using Trebuchet MS in many different Writer files, on the current version of Windows. The font seems to behave normally on the screen but if I save the document, close the document, then open the document, I find an inconsistent pattern of Trebuchet MS being replaced on-screen with Times New Roman. The font will remain correct on some documents but not on others, and in some parts of the pages but not on others. I see Trebuchet MS installed in the Control Panel. This problem also existed with at least 1 previous version of Windows and at least 1 previous version of LibreOffice. I always have the Stable version of LibreOffice. Help!

How do you know the on-screen font is Times Roman?
Is that font displayed as the applied font in the LibreOffice dialogs?
How are you applying the Trebuchet font?
Are you doing manual formatting, or using styles, or both?
Can you provide a sample document?