Confusing TK test results

A couple of weeks ago my 2012 wk2 crd (early ecodiesel) was diagnosed by a mechanic as having a bad head gasket after an egr bypass test to confirm if it was egr or head gasket (egr is still bypassed). He said there are bubbles coming from the system (which is true) however after adding k seal as per the bottle instructions I am still getting bubbles.

I did a tk test before and after adding the k seal, it passed both with no colour change whatsoever. I have verified the fluid is good by sticking the tester into the tailpipe it went green almost instantly. It has passed 6 tk tests with the engine at various rpm from idle all the way to 3500 with the bubbles passing through the tk fluid but failed one when I was doing a dpf regen in the drive (lots of smoke in the air)

Any ideas as to why it’s got bubbles passing through the cooling system but can still pass tk tests? Any help or ideas on how to fix it are greatly appreciated

The only engine codes were for a full dpf and a egr below flow which both make sense since the egr is not connected currently and it’s been idling in the drive for some time