What you are describing does not point to one definite cause yet.
Since it happens intermittently, across different applications, and seems to involve J, M, U, 1 and 3, I would work through a few simple checks before assuming that the keyboard itself is faulty.
Start by checking System Settings >> Keyboard >> Text Input >> Edit and make sure the correct keyboard layout and input source are selected.
If you have more than one input source enabled, temporarily switch to another standard one and test the same keys.
This can help determine whether the behaviour is associated with the current input source. Apple: If your Mac doesn’t respond to key presses
I would then check System Settings >> Accessibility >> Keyboard and make sure Slow Keys is off.
Also check Accessibility >> Pointer Control and make sure Mouse Keys is off.
The latter is especially worth checking because Apple uses U, J and M for pointer control when Mouse Keys is enabled. Apple: Change your Mac keyboard’s response speed Apple: Use your keyboard like a mouse with Mac
The next time the cursor jumps left, open Keyboard Viewer from the Input menu and watch what happens.
The important thing to look for is whether the affected key produces its normal character while Left Arrow is also being registered. Apple: Use Keyboard Viewer on Mac
If you have access to another keyboard, try that for a while.
If the problem occurs with both keyboards, it would not be limited to the MacBook's built-in keyboard.
If it happens only with the built-in keyboard, that would make the keyboard itself worth investigating further.
If the problem remains, try Safe Mode.
On an Apple-silicon Mac, shut it down, hold the power button until the startup options appear, select the startup disk, hold Shift, and choose Continue in Safe Mode.
Apple notes that Safe Mode can help identify problems associated with software that loads during startup.
Finally, if the behaviour continues, run Apple Diagnostics.
On your M4 MacBook Air, shut it down, hold the power button until the startup options appear, and then hold Command-D.
This can check for possible hardware-related problems. Apple: How to use Apple Diagnostics to test your Mac
The key thing to establish is what happens when J, M, U, 1 or 3 is pressed: does the expected character appear while an unintended Left Arrow input is also occurring?
That observation will help determine where to focus the troubleshooting without prematurely blaming either the keyboard or macOS 27.
