I have the same issue with many of my IG's using multiple columns of enterable popup lov's. I had not noticed this issue until recently and was well into this project. Therefore I have many IG's to go back and fix with this workaround which seems to work. I have been searching to find if this is a bug and is fixed in APEX 18.1, I am currently using APEX 5.1.4 Oracle 11. seems to be unrelated to browsers, I've used Chrome 68.0 and Firefox 51.01.
BTW I tested this issue in the interactive Grid Cookbook page 2 and it occurs there also when you change the MGR or DEPTNO columns to an enterable popup lov.