Thanks for your input, Mike.
Since this is clearly not a bug, but the intentional behaviour, I have moved the topic to the FontCreator Discussion forum.
I think, for now, I will just leave this feature as Initial Forms, since that (Word Start Alternates) works as expected in PagePlus, etc., and is easier to implement. Initial Forms are enabled by default in Affinity Designer too, if I remember rightly, so no interaction is required from the user.
See this thread on OpenType Feature to Replace Initial Forms