"What could this issue be?"
This:
{"Flow Rate", "Velocity"}
Either use a string array or a cell array of character vectors, but do not create cell arrays of string scalars (which, because it is a container array of container arrays, is not processed as text). The MATLAB documentation specifically recommends avoiding them: "Avoid using cell arrays of strings. When you use cell arrays, you give up the performance advantages that come from using string arrays. And in fact, most functions do not accept cell arrays of strings as input arguments, options, or values of name-value pairs. For example, if you specify a cell array of strings as an input argument, then the contains function throws an error."
