It's a little underhand. Sometimes the person who hired you may have genuinely forgotten that probation pay is less than full pay. The savings to the company are only a few thousand, assuming your probation period is only a few months.
The key point is that since you haven't signed anything you are still free to negotiate. Reply to the company and say "I accepted this job on the assumption it would pay the full rate from the start. Can you change the offer to make that happen?" Or ask for a small increase in your base rate to compensate. Or some other benefit, like maybe a signing bonus.
Since they have changed the offer you are free to negotiate and, if you wish, refuse the offer.
You say you are in engineering. In my experience in that field in Canada the reduced pay would be very unusual, and I personally would not tolerate it. I'm doing the same amount of work after all.
The Covid policy shouldn't make any difference. The government will pay 75% of actual salary, so it's not like they are getting you for free. On the other hand if they even hint something like "We will record you as being paid 6k but really only pay you 75% of that", then run away very fast. Not only would that be illegal, but if they are only looking for workers entirely paid by government subsidy then they will dump you when the subsidy ends. And you might be considered an accessory to the fraud.