Bug in iOS 13: the picture that you are looking at doesn’t stay on that picture
This bug you are describing is the phone not being able to keep the photos app opened in the background, which is a limitation of the phones RAM and how software handles the RAM management. However, many factors can change how many apps and which ones it can keep open in the background. This is also nothing iPhone specific, even computers - windows, macOS, Linux, i.e. every operating system - have to manage the available RAM.
Possible factors that (could have) changed for your iPhone:
- Software update: iOS is closed-source, i.e. no-one exactly knows how iOS is programmed, however, a full iOS update has many changes not only to the OS itself but also to its built-in apps like the photos app. Adding more features to an operating system usually also means your device has to handle more things at the same time, therefore some things may slow down.
- More apps open at once: If you have many apps open at once iOS tries to figure out how to keep as many open as possible, which sometimes means that one big app (especially big games) may be closed earlier than others, even if they were used later.
- Battery degradation: Depending whether you have enabled maximum performance under battery settings, your phone may limit its general performance, as the battery might not be able to power the phone under peak load anymore.
- Other factors: It is really hard to predict what exactly may have caused an app being closed. It could even be something as simple like you getting a big email that your phone is trying to download in the background and therefore needs to free some RAM.
However, if you test under as close conditions as possible to how it worked under iOS 12, you can most likely pinpoint it to iOS 13, which is very well possible.
In this case you'd need to have a backup to revert to the previous iOS version.