QR code detection and reading does not involve any sort of "extra window", "screen redrawing" or the like. The fact that many QR code reader apps for example display photos with boxes in bright colors drawn on them, etc. does not mean that such display is actually a necessary part of the process.
I have never heard of Photos having a QR code scanning functionality. It does not use QR codes to index photos.
If you want to read QR codes with your iPhone, you can use the Camera app. It will by default detect and read QR codes.
Maybe you feel that the Photos app is hiding something from you, and that you suspect that QR-codes in your photos are getting "scanned" nefariously?
Your grounds for suspicion for that is that you saw something "flashing" or "redrawn" while importing photos. As mentioned above, QR-code scanning does not require any kind of on screen display, graphics or flashing of any kind.
It is most probably just a random occurence that Photos happened to be a bit slower changing from one photo to the other there. Scanning a QR-code in a photo can be done in milliseconds - it's not something that require Photos to pause between photos. In addition, indexing of photos for searching (i.e. the stuff Photos does to make it possible for you to search for locations, features in photos, etc. - as in for example searching for mountains or cat) happens in the background after photos have been imported.
In addition to this, what would Apple gain by "nefarisouly" scanning your photos for QR-codes - I cannot see anything. I guess you could search for "photos with QR-codes" in them - but besides that, what kind of secret feature would it be for? It simply doesn't make sense.
Yes, it is ofcourse possible that evil programmers at Apple hid a QR-code scanning feature in Photos that extracts all those very interesting QR-codes we all have lying around (like routing information on that UPS packages that was delivered 2 months ago, the link to that web page with video instructions on how to assemble the IKEA furniture you've bought last year, etc) - and seemingly not doing anything with that information. However, there's really no practical way of determining if that is the case.
You could conduct a very thorough analysis of the involved binaries, which includes a enormous code base of system frameworks. It's not practical.