Created attachment 127159 [details] Single cell exhibiting the fault - after save. Have a single cell with contents /=MONTHS("2016-06-01", "2016-07-01", 0)/ after save and on reloading comes back as /=months("2016-06-01", "2016-07-01", 0)/ with field showing /#NAME?/.
Unconfirmed with v5.1.5.2 under windows 7 x64. Unconfirmed with v5.2.1.2 under ubuntu 16.04 x64. Maybe you should consider upgrading to a newer version.
You are saving in xlsx format and MONTHS function do not exist in MS Excel. For me, it is not a bug.
Closing.