Your thread title should read, "...suspected...," not '...suspect...'.
When you say, "Roncalli, suspect of Modernism," it literally would mean that Modernism suspected Roncalli of something, but just what it was, of which Roncalli was suspected, isn't mentioned.
"Under suspicion of" is synonymous with "suspected of."
"Suspect of" is synonymous with "suspected by," (to be followed with the name of the person or entity that was suspicious of someone or something else).
Roncalli, suspected of Modernism.
Here is the video you linked: