I remember when the same bug slipped into QlikView some years back, for a service release or two.
Hopefully it will be resolved soon.
The thing that makes it complicated, I guess, is that Date fields are duals, and these should go into Excel as numbers, but every other type of dual you probably want the text to go across. I'm not sure how easy it will be for the developers to tell them apart. As you say though, it was working previously.
You may want to raise a ticket on this one through Qoncierge and point them to this thread, so it can get logged as an issue.
Qlil support answered to my ticket raised yesterday with the akwnoledgement of the issue and with some workaround
1. This is a known issue for us. We have already created a bug for it: QV-12560, and the fix for this problem will be included in the upcoming 12.20 Service Release 1.
2. As for now, what you can do is:
- export the data to a csv file and then reimport to Excel
- downgrade to 12.10 SR8