The obvious answer is that at some point you previously added an ossia below, encompassing those bars, and you subsequently deleted that ossia without deleting the notes. Adding an ossia below, now, shows the same notes you’d previously added to the ossia at that location.
ok - I might have done that…
But why is the ossia truncated when I create a longer one ? - shouldn’t it stay for the new length ?
Turn on View > Signposts > Staff Count Changes. I suspect you’ll find there’s a “-1 staff” signpost at the start of the second time bar. The likely explanation is that when you previously deleted the ossia, you only deleted the “+1 staff” signpost at the start of the ossia, and left the “-1 staff” signpost intact. When you try to add a new ossia, Dorico respects the existing “-1 staff” instruction.