Environment
Package: ostrio:[email protected] (also reproducible on 3.14.x)
Internal router: MicroRouter (introduced in 3.14, replacing page.js)
Browser: any (Chromium / Firefox), checked on macOS
Meteor: 3.4.x (irrelevant to the bug, but for context)

FlowRouter.route(`/test/:_id?`, { name: 'testName', title: 'test test', action() { } });

// before
FlowRouter.go('testName');
FlowRouter.getRouteName() // => 'testName'

// after
FlowRouter.go('testName');
FlowRouter.getRouteName() // => undefined

After investigating we found that microRouter handle differently the path depending an ending '/' in the path.

FlowRouter._microRouter.show('/test/'); // working
FlowRouter._microRouter.show('/test'); // not working

FlowRouter.path always remove trailling '/' in the end of path in case of optional parameters.