Yes, unfortunately this can happen because Tropy doesn't fully support datatypes other than strings yet (the date handling isn't complete yet). For the default templates, you will create 'date' values if you originally enter the value in the metadata panel. Currently, if you enter dates (for the first time) via the item table they will always be saved as strings. So my guess is that you entered a few dates via the table. I'm afraid there is no easy way, for now, to convert the datatype via the UI. If only a few dates don't show up correctly, there are two ways to achieve this:
Clear the date field completely; then restart Tropy (or reload if you have enabled developer mode). After the restart, enter the date again (via the metadata panel). It should now have the proper date type and be formatted according to your locale.
Alternatively, you can select an item where the date is already formatted correctly; then select additional items (using cmd/ctrl click) where the dates are not formatted correctly. Then enter a date in the metadata panel, effectively changing the date values of all the selected items. Obviously this will set the same date value on all items, so you will likely then have to go through all the items again and set the correct dates individually.
We realize that this isn't an ideal solution and we'll add better ways to change datatypes once Tropy fully supports more than string values.