This is not curious, but the expected behaviour. Matlab inserts commas for separating the elements of a vector automatically.
Use + to concatenate strings:
["aa" + ...
"aa", "bbbb", "cc" + ...
"cc"]
Note that 'abc' is a "char vector", not a "string". Confusingly {'abc'} is called a "cell string", although it is a "cell array of chat vectors". The cause of this confusion is that the "string" class has been introduced in 2016 and the term "string" was used for char vectors before.
With char vectors use the concatenation operator [ ] to join the subvectors and create a cell array using { }:
{['aa' ...
'aa'], 'bbbb', ['cc' ...
'cc']}
