I know this can be a polarizing topic, but I'll be skipping the coolant conditioner in my subaru. I feel like Subaru requires it as a short term stop-gap to limp your car's head gaskets past the warranty period.
Down the road, stop leak can cause a huge headache and leave your system clogged up. My car was getting pretty warm over long uphill grades, turned out to have had bad exhaust valves and a cracked cylinder head. I replaced those, but it was still getting warm. Tried a lot of different flushes and finally got something that cleaned out all the stop leak out from all the years it was put in. Now the temps are much more manageable.
Are you absolutely sure that the ONLY additive to your system was the Subaru Conditioner / Holts Radweld Plus? The main ingredient is Borax, and it forms a glassy material where it gets out thru a void and dries up. But it's soluble, so it should leave little behind.
I suspect that someone threw something like Bar's Stop Leak or something similar into your system. I've used Bar products in cars since the 1970's. Those 'plastic' pellets are incredibly effective at sealing leaks, but if you overdo it (especially in combination with hard water), you end up with quite a mess.