Return Key Not Working in Safari

Issue: Pressing the return key does not trigger the search action. Nothing happens.

Steps to reproduce:
(1) Enter a search term/query
(2) Press the return key

Troubleshooting steps explored:
(a) Disabled all extensions to run bare Safari. Result: No difference.

System Info:
macOS Monterey 12.7.6

Apps and Version affected:

Safari 15.6.1
Result: Not working

Safari Tech Preview: 16.4 (release 156)
Result: Works as expected

Librewolf 125.031
Result: Works as expected

Expected result:
Pressing the return key triggers the search action.

Actual result:
Pressing the return key does nothing.

Follow up:

Here's what's showing in the JS Console (in Safari):

TypeError: searchForm.requestSubmit is not a function. (In 'searchForm.requestSubmit()', 'searchForm.requestSubmit' is undefined)