"might be truly" is a correct and usable phrase in written English.
It can be used in a sentence to express a possibility or uncertainty about something being genuine or accurate. Example: The rumors about the company's financial troubles might be truly concerning.
Maybe she should have held her nerve, but then the book might be truly unbearable.
Doubtless such a triumph would horrify Tolstoy, and he would not like to acknowledge that it might be truly useful.
THOMAS VINCIGUERRA From the start, some readers seemed to think Mad might be truly dangerous: Educational, entertaining, humorous?
If our ancestors channeled their collective death instinct into religious myth, we now face a raft of scientific data that suggest the end might be truly nigh.
Whereas I might be truly impressed by about a dozen out of 100 interesting-looking films, the Korean pile yielded several times that.
— Anonymous If we really seek the ultimate sources and manufacturing conditions of everything we use in our daily life, we might be truly frustrated by the cruelty of the process.
So a grad student's account of grad school might not be truly representative of what went on; it might merely combine the best (or worst) with how it all turned out.
He might even be truly sick, fucked up, in pain, who knew?
Wine's progressively intertwined shapes might not be truly infinite, but they beckon the eye toward eternity.
Let's first take sampling error, which reflects the unavoidable fact that the opinions elicited by pollsters might not be truly representative of the electorate as a whole.
But that does not mean that there might not be truly customized treatments for cancer 10 years down the line, she said.
