There are no (and never have been) rules about splitting infinitives. The placement of the adverb before, in the middle of, or after the infinitive should only be guided by the meaning that you are trying to convey. Sometimes it matters, sometimes it doesn't. The correct version is the version that correctly conveys the meaning that you mean to convey.
I currently make my living copy-editing scientific journals. This does not make me an expert on grammar or spelling, but it does make me a near-expert on using style guides. There are lots of them, and the one thing they have in common is that they contradict each other all the time. When people try to assert that their rules of grammar or spelling are correct and that others are wrong, well, I just laugh out loud. There are common basic rules of course, but I am talking about the more subtle uses of language, the ones all debates are about.
Spelling and rules of grammar change all the time, especially spelling. The progression of compound words into hyphenated forms, and later into combined nonhyphenated versions is commonplace and occurs very quickly. It's not wrong, it's not even unusual. The conventions that happen to be popular at the time that you learn them are no more correct than earlier or later ones. Get used to it.