In https://github.com/aklinker1/webext-core/blob/27195e9974a99573ff458ceef993edac0d76b1a4/packages/fake-browser/src/apis/tabs.ts:

fakeBrowser.tabs.create creates and returns a tab but it doesn't actually add it to the tabList variable, so fakeBrowser.tabs.query is unable to locate the newly created tab.