You're lucky if level 66 is the first time this has happened to you, this is more or less the norm for people who are 'current' on content. I remember the mid 40's being a bit of an EXP chokepoint back during ARR, or the nightmare that was moogle sidequests in the Churning Mists when Heavensward came out. I had to do so many of those to progress the MSQ.

Either way, this is normal. You'll likely run into this a lot when Shadowbringers comes out and you go through the 5.0 MSQ.