Here are a few options for a formal title, depending on your focus:
Option 1 (Most direct and professional):
“Resurgence of Online Disinformation Following President Zelenskyy’s Open Letter to Putin”
Option 2 (Focusing on the analysis):
“An Analysis of Online Disinformation Trends Following the Zelenskyy-Putin Correspondence”
Option 3 (Concise and journalistic):
“Online Disinformation Campaigns Intensify Following Zelenskyy’s Open Letter to Putin”
Recommendation: Option 1 is the most balanced and suitable for a formal news segment.