It may be that the mat file you load contains a variable called dtaset_id, which is a double. When you load the mat file that value overwrites the previous value of dtaset_id in your workspace.
Here I've attached a mat file with a double dtaset_id to demonstrate.
dtaset_id = '0';
whos('dtaset_id') % char
load(strcat('dataset_', dtaset_id, '.mat')) % overwrites dtaset_id with value from mat file
whos('dtaset_id') % now double
