Hi, pretty much what the question says in the title. I know it doesn’t mean I’m completely anonymous, but I was under the impression that using both Mullvad VPN and the browser would afford my data/search history privacy. But when I was on reddit on my mobile app just a few hours later, I had a very specific sponsored ad that I’ve never gotten before, explicitly related to the content I had just been researching. Is this just a coincidence or have I been affording too much trust with my setup?
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We can’t know for sure. Could be a random coincidence. I think it would be unlikely that you’d never see an ad for something you recently researched, it’s statistically going to happen eventually. I get a consistent combination of ads that have no relevance to me and ads that my brain could interpret as being targeted at me, but we’re working backwards trying to divine truth from pattern matching instead of finding a causal relationship between the events.
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One potential explanation is that the content you were browsing on Reddit is similar to the content you were researching, so the ad was served as related to the page, not related to the user.
I’ve had a similar scare in the past, where the homepage of Invidious instances frequently showed videos very similar to what I’d been watching on other front-ends/IPs, but I eventually realized that those videos/channels were mainly Tech/PrivSec-focused content. It wasn’t tailored to “me,” it was tailored to “the kind of person that uses alternate YouTube front-ends,” which did describe me.
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What are your desktop and phone operating systems?
Assuming you’re not logged into a Reddit account, can’t Reddit still see the Mullvad exit node and put you in a batch of people that the ad would be relevant to?
Like, you’re probably tech-savvy and given the current situation, an ad for some RAM sticks sounds like a large net that would potentially encompass a large amount of people exiting from the same node on Mullvad haha.
Or a gaming chair/other popular product.
Potentially a bias that makes you think that your taste is unique (while maybe it’s more common than you think)? 
Maybe your ISP’s router is also straight feeding your data to some broker, unlikely tinfoil but there are quite a few ways to link things together in our day and age.
Maybe even some not-so respectful installed apps on your phone being very verbose to the outside world?
Maybe other family members on the same network or some nice connected home speaker listening?
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I totally understand your point. And sure, maybe it is just a casual relationship. I just haven’t had in recent memory an ad that was so, so specific where I knew I hadn’t researched this topic outside of Mullvad so it caught me off guard.
This is a possibility, and makes me feel a bit better to think about. It still feels eerily specific since I hadn’t searched this specific content on reddit before, but it’s related enough to other communities I frequent.
I was actually logged into reddit at the time on my mobile device just on a “regular” profile of mine. I also have Mullvad VPN running on my phone, but when I was researching the other content it was on desktop with Mullvad browser + VPN. So yeah, if reddit knows I’m using Mullvad VPN then maybe that had something to do with the specific ad I received..?
Bias always possible!
No family members on my network, no nice connected home speaker, ha.
And truly hope my ISP isn’t feeding my data to a broker, woof.
I think that’s the main culprit.
Consider using frontends if you’re just reading Reddit. 