I had this with my MOT in January 2019 (the date becomes relevant towards the end of the story...) - I had a general "check engine" warning light on my dashboard which had been on for about six years without causing me any problems - the dealership I bought the car from said they knew what the "fault" was, it wasn't dangerous or likely to do damage, but ever time they made the fault go away, it came back after about 100 miles. Sadly that dealer was no longer open so this MOT was done at a different Jeep specialist.
For the first time, the car failed because of that warning light (change in MOT requirements since the previous year), but they told me as it wasn't a dangerous fault and I still had 2 weeks MOT to go, I could take the car away and bring it back later, which I did. That's the bit of the story that's relevant to the OP's question.
They fixed the fault, gave me an MOT pass...and as I drove off the forecourt, the warning light came back on.
Scroll forward to January 2020 and this time, for convenience the MOT is being done at a garage much nearer home. I put it in with the warning light on, and it passes. I question this, to be told...
"The warning light MOT requirement only applies to cars 2008 and newer...your car shouldn't have failed for that last year." Online research confirms this to be true.