This "scrolling to the top" issue is a known problem that began with OSX 10.10. It's caused by an Apple bug, but is now fixed in OSX 10.11.3 and higher. LockDown Browser is compatible with macOS 10.12 and higher, so we encourage you to upgrade to resolve this issue.
Apart from upgrading, the only workaround available is to use the "text editor full screen" button (if available in your LMS) for any exam questions that require you to enter text. By clicking on this button, a full screen window will open and you'll avoid the cursor scrolling to the top.