I found a way to solve this issue. Let me describe my experience.
I was having the same issues as all the people here. I've been using iPhone since the first generation. This is the first time I notice that the keyboard is so frustrating to use once I upgraded from iPhone 12 Pro Max to iPhone 15 Pro Max. I'm having issues of typing on backspace, and letters around bottom right corner.
I went to the Apple Store and people there won't be able to fix the problem. They even argued that my finger didn't touch the key. After I tried several times, and they tried on their own on my iPhone, they start to understand the issue. But they having trouble of solving it.
I did several thing on my own after reading the actions people did here.
my system is iOS 17.3.1 which is the newest while I'm writing this reply. I did following steps to solve the issue
1) reset keyboard dictation. Settings - General - Transfer or Rest iPhone - Reset - Reset Keyboard Dictionary (not working)
2) like one of the post: Turn off Enable Dictation, turn off Auto-Correction (not working)
3) follow the step 1, but Rest All Networking (won't solve the issue)
4) turn on Hapic feedback. Settings - Sound & Hapics - Keyboard Feedback - Hapic. (works)
I don't know whether we have to try all the steps. At least, I tried hapic feedback, it works. Another reply here said turning it off would do the tricks. So I think some of the issues may relevant to the Hapic and sound feedback when we are typing. Turning it on or off would definitely trigger something then fix the issue. I guess the department working on the software of this part make some mistakes or typically we said there are some bugs here.
So try your combination. I think reseting networking sometimes fix some issues we don't think relevant intuitively. Probably the combination of Keyboard feedback Reset Networking.
Leave your note here. Let's raise this up to let apple do fix the bug here. This is definitely a very very very critical mistake they ever made. Like some replies said we would return the iPhone just because of this "tiny" issue.
It's very frustrated to people like me.