Previously single quotes still worked, however it was known as a bug. As of Qlik Sense November 2017 and I think QlikView 12.20, the bug was fixed and many set analysis expressions stopped working, unless they were recognized as an older version. This would not be the case for a conversion.
There is no general design difference between Set Analysis in QV and QS. Except, as previously mentioned, where bugs have been fixed or introduced. This is more of a release rather than a product issue as the products share common engine code. For example what @treysmithdev mentioned as closing the loophole for single quotes in an advanced search. There was also a bug for some period requiring double quotes around dual values. So converting from a QlikView (possibly older engine version) QlikView to QlikSense (possibly newer engine version) you may have encountered one of these changes.
However, looking at the details of your reported problem I don't see any bugs or behavior changes I'm aware of. Instead I would validate that the conversion was complete in regards to variables etc.
Qlik Sense has a wonderful expression preview in the Expression Editor. Open the expression you reported as problematic and confirm in the preview at the bottom that the variables are returning expected values and the the syntax checks out OK.