Yes they do, John.

However we have been able to find what caused the problem, thanks to your former remark about PPT versions.

The presentation I asked my colleagues to check earlier this week was made in PPT2016 as a ppTx file and then saved in PPT2016 as a ppSx file.

Thinking about your remark and knowing I have different versions of PPT on my pc (to work in for different customers) I asked my colleagues to test the ppTx file instead of the ppSx file, and that worked just fine on all pcs.

So apparently when I save a PPT2016 file to ppSx, that Slide Show presentation opens up as a former version.

Just tested it with a completely new presentation to be sure, and exactly the same thing happened: ppTx works fine, saving as ppSx turns the Morph transition into a Fade.

So now that we know what's causing it, we can work around that

Thank you again for your answers!