According to the bootstrp documentation, bootfun should be a function handle but bootstrp merely calls feval which accepts character vectors of function names which is why there isn't an error.
In the syntax bootstat = bootstrp(nboot,bootfun,d1,...,dN), d1,...dN are the variables to be sampled where rows are observations. That's correctly set up in your first call to bootstrp. However, in your second call to bootstrp you are only sampling the rows of y while referencing the same set of x values for each bootstrap repetition.
The line below shows the correct way to set it up.
[bootstats_two]=bootstrp(10,@(ybs,xbs)regress(ybs,xbs),y,x)
Now the rows and y and x will be correctly paired.
