Sounds like peterk has identified the issue. Moan if changing your gatekeeper settings doesn't fix it. Oddly enough we are not 'recognised apple developers' (which I presume means 'paid apple money for the priviledge of writing software for their stuff' (checks: yes $99)). I am never giving Apple money for anything - they are an embarrassment even to other large venal corporations, and are pretty much the epitome of everything opposite to freedom in computing.
Isn't that lovely - your computer now comes set by default to refuse to install things unless the authors paid Apple $99. I think that's what normally referred to as a protection racket. At least it seems to be easy to turn off (for now...).
Debian, on the other hand, has been ensuring that all its apps are signed since 2002, and no-one had to pay for the priviledge, and by default your computer will not refuse, just say 'these packages: (list) are not signed, are you sure you want to install them'. Survex of course is signed there. I know which of these implementations I find fair and reasonable.
And to be clear Olly Betts does nearly all the Survex development, not me.