In this sense, shouldn’t it be hyphenated?
“She was over-careful.”
The OP has noticed one of many frightening trends about what is happening to the English languish (sic)! People can no longer tell wehn it is proper to use the adverbial form of a word! Sad. And, with Mirriam Webster accepting that computer geek text talk into our language, there is little hope for the next generation. Sheesh, imagine if terms like “10-4” and “pregnant rollerskate” would have made the Mirriam Webster in the 1970s from the CB era! Can’t the geeksters get their own dictionary, like the CBers did?