Database name mismatch happens while opening the table for
virtual column computation. Because table_name_parse() returns
the length of database and table name before converting the
filename to table name. This issue is caused by
commit 8b0d4cff (MDEV-15855).
Fix should be that table_name_parse() should return the length of
database and table name after converting the filename to table name.

Reviewed-by: Marko Mäkelä