I'm either hallucinating OR getting confused with "Off The Ground"...but I know I could have sworn Wix played on everything in that late 80's/early 90's period. I'm shocked!
Wix's first studio session was for overdubs for the single version of Figure of Eight. I believe that was his only involvement with the 'FITD' sessions even though his contribution wasn't used for the actual album.
I have the McCartney "Recording Sessions" book, and the first time it mentions Wix on any track is April 1989 for the track "Party Party," although he would be on that single version of "Figure Of Eight" as well as the alternate version of "Rough Ride" from the "Put It There" special. He then appears on the soundcheck tunes "Together" and "Inner City Madness" from the 1989-90 tour that appear on Tripping The Live Fantastic. And then of course Unplugged in early '91. The first full-blown studio album track he participates in is "Biker Like An Icon," recorded at the end of 1991. Very strange things about keyboard credits in this era. As I noted earlier, the band demos of the Paul/Elvis collaborations clearly have keyboards, but no musician credit is listed for them (maybe it's Paul?). The "Recording Sessions" book says that the keyboardist David Clayton of Simply Red spent a few days recording with Paul, and Club Sandwich mentioned him playing on "Motor Of Love" at the time, but in the end, his name appears nowhere in the credits of any track (there is supposedly an earlier, unreleased version of "Motor Of Love," so he must be on that one, I guess).
Hi, I'm Luca the author of the "Paul McCartney Recording Sessions" book that you mention. Right, to my knowledge, Wix was not involved on Flowers and Robbie joined in only later in the game for overdubbing. The Figure of Eight remake is from September 1989. The keyboards on the 1988 band demos are strangely not credited, it could be Paul himself, and possibly Nicky Hopkins on That Day Is Done.