I've seen irons with tips that work perfect for the first few days or weeks then turn horrible and require constant heavy duty cleaning every time you use them.
It seems quite random, sometimes it's the cheap ones and other times its the official brandname tip from a certified agent. Dad had a Goot iron and the tip on that was simply awful. (Perhaps these are tips that come pre-tinned with leadfree)
However i've never come across any terrible tips in the hakko 900M style, even among aftermarket versions.
The 3rd party 900M tips definitely don't last as long or work quite as well but they always stay tinned and work satisfactorily.
For hobby use, if you don't use lead-free, what you wrote might be perfectly true. With Lead free, after the first day, the best weller tip will be oxidized. Even the HAL solder on the boards can contaminate the tip.
I would still recommend to try the activator.
Yep, i've only ever used leaded solder. Luckily i've never encountered a situation that required lead-free