Changing the file name for a PDF report from Access

[Petrol]

I have an Access 365 application in which the user can print various reports as PDF files. To achieve this I have a "Print" button on the report which calls a standard module which uses DoCmd.Printout to print the report to the default PDF printer. Access sets the name of the resultant PDF to the caption of the report. While the captions (and thus PDF filenames) are set at design time to a generic name, I want to differentiate individual instances of a report by dynamically adding to the caption, when the report is opened, an identifier held in a global variable.
Sometimes this works, sometimes it doesn't. When it does, the filename comes out as the new value given when the report is opened. When it doesn't, the filename remains set to the string given at design time.

I have read suggestions on various forums suggesting the questioner should either
(a) open the report in design view, change the caption, then switch to report or print view; or
(b) Use DoCmd.OutputTo instead of DoCmd.Printout. 'For various reasons I am reluctant to use these approaches. The question exercising my mind at present, however, is "Why does changing the caption in the On Open event procedure work in some instances but not in others?"

Examples:
(1) Report 26: Caption given at design time - "All Emmaus Events".