Reports are starting to surface that some Pixel 3a users are experiencing random shut downs with their devices. This is not happening to everyone, just a small group of users. Google hasn't yet acknowledged the problem, but that will likely change in the next few days.
The next update for the Pixel 3a isn't scheduled until the first week of June. So this might be an issue that Pixel 3a owners have to deal with for a little while.
According to those that have reported the issue on Reddit, this appears to be happening when the phone is in standby. Users are picking up their phones after not using them for several hours, and finding that their phone had turned off. It does seem to be happening at random, but it is also happening more for those that are using WiFi, versus those using 4G LTE.
It's a rather strange bug for the Pixel 3a. Hopefully it's something that is fixed with the next security update for the Pixel 3a, however.
Issues with a Pixel smartphone is nothing new. Especially with the current lineup of Pixel smartphones. The Pixel 3 had a ton of issues, especially the larger Pixel 3 XL. From it randomly growing another notch on the side of the phone, to performance really suffering when you use the camera and just about anything else you can think of. The Pixel 3a, however, hasn't had a lot of issues. In fact, this is really the first bug that anyone has found on the Pixel 3a (other than those that had a bug that only affected their unit and not others). That's good for Google, but this bug is definitely not good.
Google has been hit with plenty of class action lawsuits in recent weeks, due to issues with some of its older smartphones. Like the bootloops on the Nexus 6P, and the microphone issue on the original Pixel line. It's unlikely that would happen here, but if Google doesn't get it fixed soon (or if it turns out to be a hardware issue), it could end up happening. And these class action lawsuits aren't small lawsuits either, with Google forking over billions in the settlements. One might argue, they are paying more in class action lawsuits than they are making by selling their hardware to customers.
Our Pixel 3a isn't having this issue, but that doesn't mean that it's not happening. It does seem to be happening to a small set of users, and that (as usual) appears to be the vocal group of users that are going on Reddit to make sure that everyone is aware of the issue. We'll have to wait a couple of weeks to see if the issue is fixed in the June 2019 security patch or not. But where Google hasn't acknowledged the issue just yet, that may or may not happen.
If you are experiencing this issue, you may want to take it off of WiFi. Even though it's unproven, that does seem to be what's forcing the Pixel 3a to shut down.