It appears if an 'append' has ever occurred on the TiboQuery, 'delete' will only be successful if the TiboQuery has been closed and reopened before the 'delete'.  This may be how it is designed to work, or it may be a bug.

Starting the program, TiboDatabase set to 'AutoCommit',   using the following steps:

1.  Clean database, open TiboQuery, write data, delete data:    fails.       

2.  Data in database, open TiboQuery, delete data:   works.

3.  Data in database, open TiboQuery, delete data, write data, delete data: fails. 

4.  Open TiboQuery, write data, close TiboQuery, open TiboQuery, delete data:  works.

5.  Clean database, open TiboQueryW, write data, do not close TiboQueryW, open TiboQueryD, delete data: works because it is through a different TiboQuery.