After purchasing the year 1 pass, it is not retroactively unlocking Premium Access vanity items and emotes of the Field Research Assignment Stages that I completed prior to purchasing the year 1 pass. However, it is unlocking the Premium Access vanity items and emotes of the Field Research Assignment that I have completed after purchasing the year 1 pass. I completed the 4th stage of the Technician Field Research Assignments, and it unlocked the Bat Swing emote just fine. But the other ones I can't find in my inventory. The easiest one that I thought I could show in photos would be the gunner outfit. The first screenshot shows the Field Research Assignment is complete with all the check marks. Second screenshot shows the outfits that are unlocked, the Gunner outfit is not included. The third screenshot shows that the year 1 pass is purchased already when I click on Premium Access. If these images are too blurry, I can try and do a video, but I've never captured anything before, so I'd have to do it this weekend when I have more time to figure all that out. But this should be easily tested on your end by just buying the pass on an account that has at least one stage completed of the Field Research, and see if it retroactively unlocks the Premium Access vanity item.

https://imgur.com/d1ZmOCR

https://imgur.com/gtjHSPs

https://imgur.com/2sf9plH