I am using LibreOffice 5.2 and am trying to use the input field function. I can input the field name and save it but when I reopen the document the field I have inserted is no longer there. Where the field was is now totally blank. If I insert any other type of field e.g. Date, time etc. it is correctly shown when I reopen the document.
If you mean the text Box on the Form Controls toolbar in Writer: that’s correct. Apparently the text box isn’t converted at all: in a sandbox document with a text box saved in docx format, the text box is missing (you can verify that easily: change the extension to .zip, open the archive, navigate to the word folder and open document.xml). IIRC this is a known problem.
See bug 83976.