The MLS have stated that Matuidi has been paid more than what is allowed, making Inter Miami have four "designated players" last season rather than the allowed three.
The designated players list allows the club to go over the MLS salary cap.
Ironically, this rule is known as the "Beckham rule", after it was introduced in 2007 when Beckham joined Los Angeles Galaxy from Real Madrid.
Beckham was given the option to purchase an MLS expansion franchise, leading to him becoming co-owner of Inter Miami, playing their first game in the competition in March 2020.
The MLS have released a statement stating that the World Cup winner had been wrongly categorised as a "targeted allocation money" (TAM) player.
The statement added that "Matuidi's compensation was above the compensation limit for a TAM player and he should have been classified as a designated player".
"As a result, Inter Miami CF violated the designated player limit as the team had three designated players in addition to Matuidi during 2020."
The sanctions imposed will be announced at a later date.
However, in the 2021 season, Matuidi has been declared as a designated player, with Gonzalo Higuain and Rodolfo Pizarro also being named in the list.
Due to the ruling, Miami have had to cancel the contract of the other designated player in the 2020 season, Argentine midfielder Matias Pellegrini, who is set to join Fort Lauderdale, a USL League One club also based in Miami, and thus will not be eligible to play in the MLS this season.
Inter Miami are currently managed by Beckham's ex-team-mate Phil Neville.