bug: canActivate does not cancel navigation when using browser back button #43181

Which @angular/* package(s) are the source of the bug?

router

Is this a regression?

No

Description

When using the browser back button to go back to a page with a canActivate guard, navigation is not correctly prevented if the guard returns false and the back button is pressed multiple times.

Steps to reproduce:

1. Clone attached reproduction, run npm install, and then run npm run start.
2. In the browser, you should start on /page1.
3. Click "Go to Page 2". Observe you are on /page2.
4. Click "Go to Page 3". Observe you are on /page3.
5. Click "Disable going back to page 2". This will set a flag that causes the canActivate guard on /page2 to return false.
6. Click the browser back button. Observe that the canActivate guard returns false and navigation is correctly cancelled. You should remain on /page3.
7. Click the browser back button again. Observe that the canActivate guard is not called and navigation is not cancelled. You should now be on /page1.

Please provide a link to a minimal reproduction of the bug

https://github.com/liamdebeasi/ng-guard-repro

Please provide the exception or error you saw

I would expect that pressing the browser back button would attempt to navigate back to `/page2`, causing the `canActivate` guard to return `false` again.

Please provide the environment you discovered this bug in

Angular CLI: 12.2.1
Node: 15.9.0 (Unsupported)
Package Manager: npm 7.5.3
OS: darwin x64

Angular: 12.2.1
... cli, common, compiler, compiler-cli, core, forms
... language-service, platform-browser, platform-browser-dynamic
... router

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1202.1
@angular-devkit/build-angular   12.2.1
@angular-devkit/core            12.2.1
@angular-devkit/schematics      12.2.1
@schematics/angular             12.2.1
rxjs                            6.6.7
typescript                      4.2.4

Anything else?

No response