So I was messing around with the new feature, it's quite fun.
After making a large amount of changes to one unit, I moved on to another. It was working just fine, I tried it out and it worked great. But after modding that unit a lot, it wouldn't accept the changes in the experimental tab where you input the Base64 code.
I'm assuming there's some sort of character limit, and after deleting some characters directly from the Base64 code it accepted the changes. I hope this gets fixed, it's quite a nice feature and it would be a shame if it stays limited like this.