I'm not a fan of conspiracy theories, but this very much seems like a scam.

I had an AC discount code to use (the type you get as compensation etc.).

I'd previously priced an itinerary in N class (round-trip with one stopover; AC flights only) and got a decent price. When trying to book the itinerary a few minutes later using the discount code, to my complete surprise, the whole itinerary priced in E class. The discount code was applied, but the total cost was more than double, so completely useless.

I figured availability may've changed, so I checked both Expertflyer (which showed plenty of N availability) and tried again pricing the itinerary with and without the discount code. About ten times. Each and every time it priced in N class without using the discount code and in E class with the discount code.

Thinking this is absolutely ridiculous, I decided to call AC and ended up spending 2h on the phone with AC - which was a complete waste of time. After getting past the "availability changes constantly", "your promo code may not be eligible for N class" (other itineraries in N class priced fine), "there's N availability, but you're not meeting the advance purchase requirement" (nonsense), I finally got an agent who realized I wasn't completely stupid, but despite working with the resource desk, was unable to resolve the issue.

After 2h I figured it was time to call it quits, but I'm still wondering:

Does AC arbitrarily block certain fares or availability in lower booking classes based on whether a discount code is used or not? (I've obviously seen regular "promo codes" that were only valid for Flex fares or higher etc., but if this happens with a discount code without any (published) restrictions it does seem like a scam.

Has anyone experienced something similar, and more importantly, is there a way to resolve this with AC?

This sort of things happens occasionally. I don't think it's by design, rather sloppiness.

I recently wanted to use the recent Amex promo code. Conditions clearly state that in Europe (only), code-shares are OK. Indeed it worked for a number of destinations, including the two closest to mine. But not for mine. System would just tell me that "there is no longer any fare" associated with the promo code.
I phoned and the agent was able to issue my ticket, after an hour or so on the phone, talking to fares and to his supervisor. I checked and the code was indeed used. (Not long ago I had a similar situation and I got the fare under the code but afterwards the code remained available; no, I did not use it a second time...)

I know it happens more often than we think, especially with deep discount K/A fares. So the system takes the next available higher fare (same letter code or higher) and discounts that. Especially true if you are booking when AC issues these "sales".

The idea, imho, is that the fare is already a discounted fare, and therefore not eligible for double discount.

Never saw it with N/P fares before, that would be a first (though theoretically not impossible). How much is the fare at and for where?

The case I related above was a P fare.

Yes. An AC agent can do this manually.

Whether AC wants to, or whether they are allowed to do it is another story completely.

In my case it really appears the thing was a mistake. The code worked fine for two very similar destinations with very similar prices and similar routes. All P, prices within less than 100$ from each other.

There's that old saying about malice and stupidity.

I'm not sure it applies to Air Canada.

Lots of stories like this about the AC codes on the passanger rights facebook group.
People have no idea that its off base fare only, and in some cases people posted pictures where the code actually increases the fare.

What happens when I try with P is that is knocks off some routings. For eg can use if go via LHR, but not via FRA even those routings are the same price without the coupon

That's probably because only connections on other airlines if codeshared.