I have found another and in my opinion better workaround until its fixed!
When you switch to another locale, instead of adding in data and then saving to reveal the “Fill in From Another Locale” link, just hit refresh instead. Hey presto, it’s copied the unlocalized data over already and then reveals the Fill in From link.
My hunch is when you switch to a locale it does copy the unlocalized data over at that point, but perhaps the page is rendered before its saved. Refreshing then shows the data. So when I fill in the internationalized field and save, the original data is still there because there are no empty fields to copy back. But strangly if you switch to a locale, refresh to get the data, then do not save but go to another locale and back, the one you just refreshed is not saved. Its empty again. So you must save it after the refresh. I guess the data is only populated in the browser and just not filling in the fields until a refresh, weirdly.
Of course, there is still a risk here that you do accidentally fill a field or click save before refreshing, in which case it copies all the empty data back. I bet you’ll do it on the last of the 20 locales you have 
Edit: darn it, Sevbauer beat me to it 2 weeks ago on Github Non-localized fields data loss when filling from another locale · Issue #18942 · strapi/strapi · GitHub