Hi Surendranath - welcome in the Forum!
Honestly - I don't know how you could do that on a normal way in QCAD but if you have only a couple values to edit then you could work with different sizes of Font Heights.
What you can see in the first line is a continuous Text Height of 1 (your problem case
The second line is also with a Text Height of 1 - EXCEPT
the space between the characters. The empty space has a Text Height of 0.25!
This is not really a nice solution but in a case of need ...
1 mm with a "Short space" is the correct spelling! Provides QCAD a "Short space" and I just don't know it?