Some steam achievements are simply not registering even if done multiple times. This particular achievement called "Post to post" requires you to "Deliver an assist as goalkeeper". I've been doing this so many times, mostly in 3v3 mode and sometimes on 5v5 ranked mode. You've got a proof here of me doing it while at the top of the screen you can see the game notice me passing and scoring an assist as a goalkeeper.
#Achievements on steam!
runic pier
There are other achievements that I've been struggling with and had to do them multiple times but after few times they did in fact register and I got an achievement but the one above just refuses to be done.
Here's another clip of it not working even if the game noticed that I got an assist as a goalkeeper
halcyon comet
Same issue here, with this archivement and with the acrobatic goal too