For context, I don't use webtoons, and I'm not salty about my audience size. I just find webtoons incredibly shady.
I've been pretty suspicious of Webtoons for awhile now, there's a lot about their platform that doesn't make sense to me. They seem to be spending a lot of money, both through advertising and paying their featured creators (through patreon, which is fishy to me, why not pay their creators directly though something like PayPal or Stripe?), but there doesn't seem to be any way that Webtoons generates its money. Why is Naever pumping so much money into a webcomic platform that doesn't make any of that money back?
On top of that you have tons of fishy things happening with viewcounts like what you've experienced.another example is what happened a couple months ago, when all of a sudden (after webtoon raised the bar for its patreon support program) lots of people started losing hundreds of subscribers over night.
Just as weird (but no one complains about it) is that lots of, lets face it, bad comics, that are uploaded to Webtoons seem to get wild amounts of attention as soon as they're uploaded? How? Why? Some people will say it's because Webtoons has a larger userbase than Tapas, but you know what has a larger userbase than Tapas AND Webtoons? goddamned YOUTUBE, and when garbage gets uploaded there it doesn't find ANY viewers (granted youtube has a very different algorithm and search function from webtoons and tapas, but like, how much is that algorithm on webtoons actually helping people?)
The engagement thing is normal, just like @shazzbaa said, but to me the weird thing about your situation is how abruptly everything stopped?
My theory, is that Webtoons has been using bots and/or click farms to inflate its userbase to rise up the ranks of various mobile app stores, and then it uses that seemingly huge userbase (and its deep pockets) to attract content creators to its website. Back when the unsubscription thing happened, they realized that they were spending too much money on their featured creators because of those bots, so they had them all unsubscribe from those creators to knock them down a couple notches on their patreon support program.
That said, I don't know what's happening with your situation specifically.