I discovered that if I open a text file which is in archive in Opus and save it, changes are lost because it's saved to temp file.
For example:
create a new empty plain text document
archive it into zip
open (with Opus) that text document within archive in for example Notepad
type something like "test" and save, close the Notepad
observe that the text file in archive is not updated, hence loss of data
If I do this with for example 7-zip file manager everything works OK (file manager detects change and asks whether to update the changed file in archive).
I waited over the weekend but still no reply from GPSoft (even though they were active over the weekend here but I decided to wait patiently until the weekend ends). They could at least say "we're still processing your inquiry, please stand by"
I think they've just forgotten about this/don't care (maybe they haven't but that's the idea I'm getting).