I run an instance in Brazilian Portuguese and I feel the same. There’s a recent topic about this and I mentioned a suspicion of mine.
I just found an example:
Notice that on Feb 12, 2021 the Bot removed the old translation because the english string had a “these” changed to a “this”.
I think ideally we’d keep old translations if the change isn’t significant, like gettext does. A wrong plural is better than the whole string in English.
I went ahead and looked further about the string on your screenshot, and its key has been renamed because of a code refactory. I assume this is an exception, I guess most missing translation cases that existed previously are caused by a change of the original text.