Photos not seeing new pics on iPhone using iOS 26.

Certainly the connection with iOS26 sounds suspicious, but there is no barrage of similar experiences being reported. So what's different about your phone or Mac? It could be 3rd Party software like, as Matti suggests, cleaners, file sweepers, virus vaccinators or a VPN. They could be interfering, because they haven't kept up, or because they're just garbage programs. Apps like those are known to corrupt files and structures.

How much free storage do you have on each device. Apple recommends at least 10% of your storage be kept free-- weird things begin happening, otherwise.

You should do a forced restart on the phone:

Force restart iPhone - Apple Support

This is different from a simple Power Off. I had to continue holding the button down past the "slide to power off" message, and then I released the button only after  the Apple Logo appeared indicating a re-start. 

And on the Mac, to narrow things down, try these things:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

Let us know…

Maybe there is pocket lint in the iPhone USB port?

https://talk.tidbits.com/t/cleaning-the-iphone-charging-port/31893

Do you use a known-good USB cable that truly supports data transfer? If in doubt, try Apple brand cable and try again.

Does it work if you use /Applications/ Image Capture.app instead?

Also don't use any 3rd party virus apps or "cleaners" because they often corrupt the system and are not needed.

Do you transfer picture from the Mac to the iPhone? Those can be difficult on the phone, because Photos doesn't really have full control over them. The import process in Mac Photos works to avoid duplicates, but transfers can create versions with different formats or different sizes, so they seem like different pictures.

Did you try Apple's Image Capture in the Applications folder, as Matti suggested?