There are some discussions here on the Discourse forum about this exact situation.
What is happening is: the dates in Ancestry were entered in a non-standard format. And so Ancestry chooses to mark them ‘text strings’ rather than date values in its GEDDCOM export.
Typically, Gramps CAN parse those non-standard Date formats and standardize them as Date Values. But since they are marked as ‘Text strings’, it is being told to not do that. (You can test a particular format, Find a sample and open the Event Editor, then clicking in the date field and press Enter. If the date converts to your preferences Date Display Format, that proves that Gramps recognizes the old format.
Now the trick is to tell Gramps to do this task automatically instead of manually.
Fortunately, you can automate this with a SuperTool script or with the Dates gramplet. (Both are addons from the Isotammi project.)