Forza Horizon 4

EDIT/WARNING: Note that you may want to take care not to complete too many regular PR challenges until you get this achievement. It seems that this may not unlock if you previously surpassed the seasonal goal on an event before the seasonal challenge went live and you could glitch yourself out of earning this achievement. Please see comments for more information.

These are pulled from the same pool as regular PR challenges (Speed Trap, Danger Sign, etc.) but with an additional requirement beyond the three star requirement attached. Only one (or even none) may be active at any given time, and what is active seem to rotate within seasons as well.

Obviously you'll need to have gotten through the first four seasons and joined the Horizon roster before these will appear, like 90% of the game's features.

The easy way to see if one is active is to go to the map, hit RB and select only Seasonal PR challenges. Then scroll around until you see a pink icon lit up. If you hover over it you'll see the 1, 2, and 3 star requirements followed by a line underneath with the seasonal requirement. In my case, for example, a danger sign that had a 550 ft requirement for 3 stars had a 750 ft requirement for the seasonal version. These requirements tend to be very difficult; I barely made it at 751 ft when I unlocked this, with a fully tricked out X 999 Bugatti Veyron!

As always, rewind to your advantage and if it's too difficult, wait until it rotates to a style that's more achievable to you.

EDIT (2018/10/12): It seems that this achievement is very finicky. If you're having problems, please read the comments for others' tips.

EDIT (2018/10/21): @Swooshiain: "The Playground Games community liaison has acknowledged that they are aware of the problems with this achievement, and they are looking into it."

The player FragiPL found this awesome solution and can confirm that works like a charm.
Well, based on the FragiPL solution, step by step:

Put the console offline
Reset console (restart)
Change the date to 21 October 2018
Start the game offline* and the Roman mile PR Stunt should appear. Just beat it. (Its a speed trap located at top left on the map)
Put the console online, start the game and achievement should pop up.
I used the gamepass copy.

Hope that helps.

*Update: apparently some people need to set home console in order to run the digital copy offline.

*** PLEASE READ THE WHOLE SOLUTION INCLUDING THE ::EDITS:: FOUND AT THE BOTTOM ***

For people saying this achievement is glitched or missable...

I can confirm it's not missable (for me...)**

I got it today with the Autumn danger sign, I had already 3 starred the sign a little over the 550ft

I went back to my home, and modified my Agera RS to the max, and gave the gearbox more speed, and the rear wing on full cornering for aerodynamics

After this went back and run the Jump again, smashed the 749ft that it needed and achievement popped...

Now if you beat the jump by over 800ft,.. My guess is you need to beat your own personal score for the achievement to pop***

*EDIT:: The Seasonal PR Stunt have to be unlocked before it can be accessed
Autumn Stunt Name: Roman Ruins
Location: Top left of the map
Car to use: Agera RS fully modified or anything with grip in X class (Porsche 918 Spyder also another beast of a car in x class)
Winter Stunt Name:
Spring Stunt Name:
Summer Stunt Name:

**EDIT: this might not be the case for you... If you have beaten the PR stunt with a higher score than the SEASON Objective YOU WILL NOT GET THE ACHIEVEMENT FOR THAT SEASON

***REPEAT::
IF YOU HAVE BEATEN THIS WITH A HIGHER SCORE THAN THE AUTUMN RECORD (BEFORE THE SEASON STARTED) BEAT YOUR OWN PERSONAL SCORE... AND SEE WHAT HAPPENS

(THIS HAS NOW BEEN CONFIRMED AND ACHIEVEMENT WON'T POP)

As other posters have said, this needs to be flagged as bugged/missable until Playground issues a fix. If you have the seasonal objective complete when a new week starts, you will not receive the achievement and no amount of beating your personal best will pop it. You have two options:

1. Save Scum (not recommended)

Delete/backup your save file, start fresh, work towards unlocking seasonal events and do it fresh.

2. Time Lord (recommended)

If you're on PC (no way to confirm if this works on Xbox), change your date to the following week and then boot up the game. Seasons are tied to your local date and time, not Playground's servers. Keep going forward until you find a stunt that you haven't fulfilled the seasonal PR objective for. It will be red instead of gold, and has a higher requirement than the 3-star rating. As a safety precaution, make sure your game is in Horizon Solo and isn't attempting to load an online session. I had to go 3 weeks ahead to find an event I hadn't already fulfilled. Once I completed the seasonal objective, the achievement popped immediately. I reverted back to standard time without issue.