ser -> a mostly permanent characteristic
estar -> a current characteristic that could change in the future
estar a ser -> a characteristic of the specific interaction
Brazilians thus have "estar sendo" which can be used in many cases where English has "is being", but as a non-native speaker I'm not sure what I can claim about its connotations or whether it's the same as "estar a ser". But my initial guess is that the Portuguese say "estou a ser" just as they say "estou a falar", while Brazilians say "estou sendo" just as they say "estou falando".
Spanish-speakers in turn have "estar siendo" with the Spanish present participle of ser, like "estoy siendo". I suspect this is the standard form of this construction in all kinds of Spanish.
"O comboio está a chegar."
(plus different phonology)
"I am tall" -> "sou alto" (ser)
"I am broke" -> "estou falido" (estar)
"I am being boring" -> "estou a ser aborrecido" (estar a ser)
What english doesn't have as easily without context is the distinction between the first two forms because it has a single verb for both.