Presumably this has something to do with the different licensing systems used for Cubase 12 and Dorico 3.5 and its effect on the new HALion plug-in. Presumably @Ulf or @PaulWalmsley will check in soon with a more definitive answer.
I think the problem here is, that Dorico 3.5 comes with HALion Sonic SE 3.5 and Cubase 12 now installs the new HALion Sonic 7. Of course, Dorico now also sees that new version and uses it, and hence the mentioned alert pops up.
In Dorico 4 you can resolve this by setting in the preferences the preferred HALion plug-in, so you could tell it to still use the old HSSE 3.5 even if HALion Sonic 7 is around. Dorico 3.5 does not have that option.
I don’t know what to suggest in this case. You are using the trial of Cubase 12, are you convinced and want to buy it or more likely not. If not, it would be easiest to uninstall Cubase and HALion Sonic again.
Actually, I really like the workflow in Cubase, so it is pretty likely that I’ll buy it. Not 100% certain yet, because it has only been 4 days so far…
So if I do buy it, will it include a version on HALion Sonic 7 (non-trial), which Dorico would then be able to use (without warnings) ?