Sorry to hear you are encountering problems with date entry. The forum posts here mention bill dates and journal entries, but we cannot reproduce any date entry problems in either of those windows. Can you describe for us what date field in a particular QBMac window where the date field clears out on you? If you could provide specific steps for what window you go into, do you tab into the date field, type the date or use the date picker, tab out of the field, return, or click somewhere else in the window to get out of the date field. Steps like that.
Also, in your Mac's System Preferences > Language & Region preferences panel, what is the 'Region' selected? For example, does it say, "United States" or does it say, "United States (Custom)". If the word custom is on the selected region, select the 'Advanced' button and select the 'Dates' tab panel ... what format is show for your 'Short' date format? Specifically, what does the format look like? Is it something like "01 / 05 / 17", or is it "17 / 01 / 05", or "17 / 05 / 01" ?