Header and footer translations deleted on save

We have a view with header and footer text. The format is set to Plain Text. In our multilingual settings we are allowing Plain Text strings to be translated.

We translated the string (into Japanese) and it is working perfectly fine.

However, when editing the same view and saving again (without touching header or footer) - a warning error message appears:
"string .. is not allowed for translation because of its text format." and then the translation disappears leaving only the source which requires translation again.