It absolutely is a real issue - it happens with my Pixel 2 XL all the time. I believe it would be one or a combination of these 3 causes:
1. The small 4gb ram which limits the number of active applications
2. Google have designed the OS to be very aggressive with ram management
3. It's an unintended bug
If the issue is # 1 , then there is no solution unless Google do some extensive OS reworking to free up ram from the OS level. Google can't retrofit another 2gb to the phones as they're already shipped, so I'd say # 1 would be the worst case scenario - 4gb ram really not being enough for the phone.
For # 2 I'm leaning to this being the actual cause. The Pixel 3 phones have quite small batteries and having the ram loaded with apps would drain the battery. Having 6gb ram would give more overhead for more applications to stay in the ram, but the small batteries on the phones mean worse battery life. I think because Google put in relative small batteries in the phones, they knew they couldn't have them running in the background continuously, so they have an aggressive ram management setup designed to kill of apps very quickly in an effort to conserve battery life. Google could have alleviated this by adding larger batteries to the phones, but instead, they decide to reduce the battery life on the Pixel 3 XL from the Pixel 2 XL.
So instead of Google putting in 6gb ram, instead of Google putting in larger batteries, they decided to have small batteries in the 3 and 3 XL and tweak the OS to have aggressive ram management to ensure battery life isn't terrible - but it's at the cost of user experience.
# 3 is the scenario that I actually hope is the case - it's just a bug, an oversight that Google could identify and fix.
Now, I don't know definitively what the issue is, I'm just speculating that it's issue # 2. Either way, something needs to be done, because as it stands, it's quite terrible. Many reports of music apps being killed off when taking photos, videos showing app reloads when just cycling through 3 common applications etc. I constantly experience many app reloads during the day on the Pixel 2 XL and this really does concern me with the Pixel 3. For a minimum $800 phone, I don't believe it's acceptable.