This issue is a little bit similar to MDEV-17226
When selecting a large number of string characters witih an UNION the data gets truncated.
I did not found any variable which can change this behaviour and I did not get any answers on my stackoverflow question regarding this:
Example is attached.
SELECT 'LARGETEXT' works fine.
SELECT 'SHORTTEXT' UNION SELECT 'LARGETEXT' truncated the text after about 34797 Bytes.
The order of the columns does not make any difference. So also
SELECT 'LARGETEXT' UNION SELECT 'SHORTTEXT'
This seems to be critical for me because the user is not noticed about loss of data.