The only way I can delete a record is to open the Base-File, load the table and use the menu “delete record”.
All other ways I have tested did not work. The buttons on the bars I use to manage records are always inactive.
I think it is be a bug but couldn’t find any helpful support.
Libreoffice 6.0.3. Linux-mint 19.1 cinnamon MySQL Connector 1.0.2 MariaDB on a Synology Disk Station
Edit 2019-05-18 from not an answer answer (by @Ratslinger):
I’ll give you one simple example:
You sea a tablecontrolfield in a form with one simple table. The navigation bar is added in the same form.
The bottom to delete a record is never acitve. But for the from, the permission is given.
When I open the base-file, there is a menu to delete a record an it is active.
I used the same form with Ooo and Windows an I never had this problem.