Yes, bonnie lass, we do seem to have to make a distinction between production/filming and transmission.
What it looks like happened is that Series #8 was produced in 1984 and went out early 1985. Foggy left to paint china eggs at that point.
In 1985 BBC commissioned and produced the feature length "Uncle of the Bride" introducing not only Seymour but also Glenda and Barry and, most significantly, Edie played by the venerable Dame Thora Hird (though I think it was later she became a Dame).
Broadcast 1 January 1986, that was well received and Series #9 retaining the new characters was commissioned and filming and production happened in 1986. An additional complication was apparently initially six episodes was commissioned but later another six were added and, of course, it is that split commissioning of the twelve episodes total that some use to justify calling the package Series #9 & 10. Somewhere in there the Special "Happy Christmas, Father Christmas" was also produced to go out on 28 December 1986 followed by Series #9 in early 1987. As Robert says, Series #9 does have 1986 Copyright date but BBC often uses production year as opposed to transmission year for that.
Then, of course, some time after production of Series #9 and the Special, probably in 1987, the second feature length episode, "Big Day at Dream Acre" was produced, to go out 27 December 1987.
Well, it all makes sense to me which was never the case with a 1986 transmission year for Series #9.