Missing texts when switching from Samsung stock SMS app

[ddoouugg]

I have been using the stock Samsung texting app on my S7 Edge. If I switch to another texting app (I've tried MySMS and Textra and Facebook Messenger for SMS, among others), many of my texts (including current texts) are missing. When I switch back to the Samsung app, I can see them again. So, they have't totally disappeared, but for some reason they can't be viewed in other texting apps. (Plus, MySMS is not backing them up to Dropbox -- as it used to do -- even when I'm using the Samsung texting app.) Any idea what's happening here?