It’s probably a coincidence or just the worst advertisers on the planet. Why would I show an ad to someone who literally just bought my thing?
The majority of targeted advertising I see is for items I just bought. There’s also a reasonable explanation for this...
Not every merchant shares purchase data, so if you do any research anywhere before making a purchase, they know you are interested, but as there was no tracking beacon on the site you ultimately bought from (or the purchase was misidentified or delayed or any number of reasons), that’s why you are now seeing targeted ads for it.