While waiting for their support to answer… I am able to repeodude this ob all of my wpml plug-in installations… latest versions of everything.
Basically: i have a string. "Hi!" In my homepage, so goes to the domain gutenberg-123
I go to the translation string section of WPML, make the appropriate translations.
Then I go back to gutenberg, and there i change the string to "Hi!!!" and save.
I see that in the section of WPML the original string appears ad changed. All good.
If I then go back to the visual editor, and there change it back to what it was before, hence just "Hi!" and save….
… in the translation string section of WPML it disappers (and in fact the string in domain gutenberg-123 gets diminished by 1)
Has anyone ever noticed that?