Not all brain lesions are symptomatic. Lesions can occur (and frequently do) in "non-elegant" areas of the brain, areas that say don't serve a vital function neurologically so when they are damaged you don't feel a thing. That's not the case for spine and brainstem however.

"Active" lesions meaning ring enhancing lesions on MRI keep that appearance for up to 6 weeks after that enhancing quality goes away. But lesions that are older but haven't completely re-mylenated can still be somewhat active clinically hence the reason why some of our symptoms dont completely disappear after a relapse or can increase with heat, infection etc. (But the term "active" lesions generally refers to MRI findings not clinical findings are therefore prob refer to enhancing lesions.)

Remember also that MS drugs arent shown to reduce relapses by 100%, more like 45-50% so people can still have relapses on a drug, doesnt mean necessarily drug failure. So even on a drug or still having some lingering effect after stopping a drug relapses can happen. Also not sure if that idea of Tecfidera staying for 3 months is proven. The drug itself has a 12 hour half life which is why its taken twice a day. So after 24 hrs a dose is gone from your system. Lingering effects are more related to your immune system taking time to ramp back up again. So, the drug holdiay might have been enough to allow your immune system to ramp up enough or it would have happened anyway even on the med cause we know it's not 100% preventative anyway. Point being there are A LOT of variables and moving parts, not always straight forward.

I'm on Rituxian every six months. Seems to be working.