The New York Times broke the story (link here) about how, ignoring his own lawyers advice, Trump reportedly spoke with at least two witnesses about what they discussed in private, confidential conversations with Mueller's prosecutors.
With one witness, Trump allegedly asked an aide to issue a false statement to the media claiming that he never tried to fire Mueller when, in fact, he actually did attempt to. With the other witness, his former chief of staff, Trump tried to find out how the interview with prosecutors went and whether they were "nice."
Some legal experts consulted by the Times speculated that such actions did not rise to potential witness tampering, but without knowing all the details, nor whether there have been additional such incidents, concluding whether a crime occured would be premature.
Nonetheless, Mueller's people are now following up with other witnesses to see if Trump attempted to discuss their interviews, too. And Trump's behavior with witnesses, even if it doesn't rise to a separate crime, can further establish proof of potential obstruction of justice by Trump.
For their part, the Washington Post broke an entirely different story yesterday (link here) about how Mueller and his team have gathered evidence that parties representing Trump held a secret meeting with Kremlin officials in Seychelles, an island in the Indian Ocean off the eastern coast of Africa.
This meeting was held to discuss a secret backchannel directly between the Trump administration and the Kremlin that would circumvent national security channels. In other words, once established, the Trump administration could directly interact with the Kremlin without the State Department, the Pentagon, or national security agencies knowing anything about it.
One participant in this meeting has testified before the grand jury working under Mueller to investigate this matter and provided further details.
It is not known whether this backchannel plan had anything to do with the secret backchannel that Trump's son-in-law, Jared Kushner, was trying to establish with the Russian ambassador to Washington, a situation that came to light in 2017 (Details here).
Both of these developments will almost certainly be reported on further in the near future. Stay tuned.