He didn't get a month trouble free, he just didn't feel it slowly blocking DPF until it got to critical level, at which point it went downhill very quickly, according to OP.
Exactly how quickly Fault Lights come back on depends entirely on the ECU programming strategy, some Cars can take a few hundred miles before Light will come back on, some cars straight away...................
Yes, if you re read it after a day or 2 you may see the fault as a "pending", but most ECU's need to see this kind of fault a few times before the lights come back on.
This is why so many Cars bought at auction do not surface faults until they've done a few hundred miles, because Vendor turned the lights out !!
Yes it is speculation on my part, not having had it in my Workshop, but this stuff is what I do day in / day out. On most DPF equipped Cars any pending or actual code which affects air flow through the engine will halt auto regen until the fault is repaired.
Sadly most manufacturers will not release there ECU Strategies, and bearing in mind that VAG Emissions Scandal I expect they have all been deleted / burned