Clutch slipping for half of a second sometimes

As mentioned, the gearbox fluid should at least be the correct grade and level etc from the dealer. I would take it back and ask them to check the clutch. Just tell them that after the service you've noticed the clutch slipping a few times. It may just be that they've taken too much slack out of the cable whilst adjusting it, this is just a 5 second job to put right. You can even check it yourself, just look up the lever adjustment recommendation in your manual.

If the oil used is not JASO MA classified what you're experiencing is one of the first signs you used the wrong oil.

Depending on miles and how hard you rode the motorcycle your clutch could be fried as well. Hard to believe on a 250R but certainly possible with enough effort.

While unlikely as @Orb mentioned the cable may be the problem. If it's been happening for 2KM and you've only had the oil changed in the last 1KM I would guess your clutch plates are bad. It's unlikely the cable fell out of adjustment that quickly and you'd likely notice immediately if you used the wrong oil. Certainly before 1KM. A misadjusted clutch cable would mean the problem would be reproducible (assuming that the clutch cable was not damaged).

I would first call your dealer who serviced it and double check if the oil used was JASO MA classified. If that is the case, inspect your clutch cable. Handle the easy stuff first. Failing that the clutch pack is likely shot and you'll need to replace it. I wouldn't think it would be the gearbox because if your shifting forks were bad you would have real trouble getting it into gear. Luckily with a Hayne's manual and some patience the process of replacing your clutch pack is relatively painless and you can save quite the sum of money doing it yourself.