Here's what I found. Looks we can go beyond the expiry date on the letter.You can continue to work until the date mentioned in the WP-EXT letter, as long as you maintain the conditions of your previous work permit, while waiting for the decision of your BOWP. The letter literally explains this clearly.
https://www.canada.ca/en/immigration-refugees-citizenship/services/work-canada/permit/temporary/extend/after-apply.html
After you apply
If you applied online to extend or change your work permit
After you apply online, we’ll send you a letter in your account (WP-EXT except PGWP) that you can use as proof that you’re authorized to keep working, if you’re eligible.
- You can print this letter or show it with your current work permit.
- The letter is proof that you’re authorized to keep working under the same conditions as your original work permit until we process your application.
- You can keep working past the expiry date on the letter until we process your application if the conditions are the same as on your original work permit.
Your letter automatically lists an expiry date that’s 120 days from the date we receive your application. The letter expires in 120 days because
- it’s the service standard to process this type of application
- many employers and health insurance providers will ask for proof that you’re allowed to work
- an expiry date helps satisfy their concerns about your status and authorization to work in Canada
If we don’t finalize your application by the date on your letter, you can still keep working until we make a decision.