There are lots of extremely similar posts but I'm not sure if there are any exactly like this.
I changed the build number but kept the version the same. I uploaded it and now it shows two builds in iTunes Connect's app prerelease builds as expected. But on my internal testers' devices, it doesn't show any updates.
It's been a few hours now and still no change.
Had the same issue. If you turn off beta testing in iTunes Connect and turn it back on again, the builds in the TestFlight app will update, and users will get the notification. This way you don't need to change the version number.