I just updated this plugin.  I deleted the config file before updating.   I was using this mod without issue for awhile before updating. 

After I updated, I cannot get it to honor the stack sizing by category (/oxide/plugins/) but if I edit individual items (/oxide/data/) it works as expected.

That is the way it works now. If you change the Config "category" to a set number you will find that some items in that category will not change, So the best thing to do is set a category size then go through the Data json file and set the items individually.
I did not delete my config when this plugin changed and saved it to a "new text file" then pasted it into the data json file, it saved me a lot of editing.

You can use the "/stackall default" command to change each item to their set config stack sizes. That or you could regenerate a new data file.