@OP -
It's not a bug. And what you're essentially asking for is a nerf to exploration payouts or a special coefficient to credit = rank up that is lower / nerfed for exploration only.
The passenger missions giving exploration rank faster are working precisely as intended. You can make the opinion you don't like the speed of exploration ranking now being on par with trade, but unless you're also calling for a nerf to trade ranking, all the passenger missions have done is -finally- give the exploration track a reasonably decent way to make credits.
And when you make those credits, it ranks you now faster because you get a lot more credits.
Hence the only way to slow this down is to either nerf the payout amounts so exploration goes back to sub-bottom feeder level, or apply a special coefficient to rank up where 1 credit earned by exploration is no longer the same as for trade.
Further, the elite in days example is an extreme, even if true. If you do just the local sight seeing missions (under 100 LY) and not the 10k+ LY ones, it takes you quite a bit longer - because again the payouts are far higher for the longer range trips. And yet even those payouts are still far lower on cr per hour basis than most other activities. So exploration pay has gotten much better, true. But far from OP.
Bottom line, intended or unintended, all you're calling for is a nerf to exploration revenue when finally, after however long people have been crying for it, the payouts got reasonably decent.