Someone redefined "Central America" while I wasn't looking.
Guatemala, El Salvador, Honduras, Nicaragua and Costa Rica were always the definition of "Central America," as documented on the national flag of at least two of those countries.
Panama was part of Colombia, as late as 1903, hence part of South America. Still considered such.
I’ve never heard it considered as part of Central America until I read your post. Chiapas used to be part of the onetime republic which included nearly everything between Columbia (Panama) and Mexico (minus Chiapas).