I am assuming that you have a .fig file saved from MATLAB R2015a which you are trying to open and save to .eps using 'hgexport' in MATLAB R2018a. Can you post your code here to clarify this?
As per my knowledge, this is not a compatibility issue as I was able to successfully save the file as per the procedure I mentioned earlier.
Following is the snippet I used to generate a .fig file with subplots from R2015a:
myFig = figure;
Following is the snippet I used to convert the .fig file to .eps in R2018a:
myFig = openfig('myFig.fig');