Unsure why but my main scrap recyclers aren't being upgraded (other recyclers processing other items are though).
When Ctrl+Shift+Q'ing them I get:
Recycler - Normal quality
The next credit provides a 2.69% chance of upgrade
Credits used on upgrade attempts: 3.38

However, this text is the same on all of my 30 scrap recyclers and none of them have been upgraded even though some have over 200k items processed.
And while I was typing this out, I noticed a single scrap recycler that has been upgraded to uncommon with the follow log lines:
Recycler - Uncommon quality
The next credit provides a 1.17% chance of upgrade
Credits used on upgrade attempts: 0.34

So it looks like all of the recyclers have been merged into 1 entity according to the mod, I thought it could have something to do with them having been placed with a single paste action but their placement doesn't seem to indicate it (it's a bit of a mess so they are weirdly placed).
It having to do with interference from another mod also seems likely but since none of the other recyclers are affected, it would be odd.

I'm willing to share my save file if you'd like to take a look at it. How would I be able to share it with you?

Whoa, that's pretty unique; yeah the save file would be super helpful to debug. It definitely sounds like an issue with entity tracking. I sent a friend request on discord so we can chat, that was a good idea on how to coordinate and swap saves. Thank you!

Something I would be curious to test, does running '/quality-control-init' in the console fix the tracking issue? That rescans all machines. Although even if it fixes the specific issue, I'd still like to debug and see how the mod got in that state to begin with.

As mentioned on Discord, I ran the command but it didn't seem to help (though maybe I can't see the difference with my current setup).
For those interested, save has been shared, will try to update the topic once reason is known.

tldr: No issue, it's intended behaviour.

Due to recyclers recycling different items, it can lead to jumps in upgrade credits.

For the set of recycles that upgrades more often:
- Start recycling something really fast (gears) and get a high count (3200 gears == 100 seconds of productivity)
- Switch recipe to something slow like concrete, the count stays high 3201, but it looks like 100x the amount of work has been done
- When quality control inspects the machine, it gets credit for the current recipe * count completed, so a huge bump in quality upgrade attempts

For the set of recyclers processing scrap 100% of the time, the math would be:
- 0.5 crafting speed -> 2.5 / sec scrap
- So scrap recycling is 5 scrap per 1 second of crafting time
- 3 hours per upgrade attempt == 60 seconds * 60 minutes * 3 hours == 10,800 seconds needed
- 10,800 seconds * 5 scrap == 54k scrap processed per 1 full upgrade attempt
- For an entity that's processed 307,000 scrap, we'd expect 5.68 upgrade credits earned (100% scrap recycler math thus seems okay).

Recycling some concrete in the scrap recycles thus has a high change of upgrading the recycler instantly.

Thank you Oniguro, you are awesome!