Foreign key isn't being enforced
As the relevant docs say (in section 2. Enabling Foreign Key Support):
Assuming the library is compiled with foreign key constraints enabled, it must still be enabled by the application at runtime, using the PRAGMA foreign_keys command. For example:
sqlite> PRAGMA foreign_keys = ON;Foreign key constraints are disabled by default (for backwards compatibility), so must be enabled separately for each database connection separately.
Have you used that PRAGMA in the relevant connection? (Assuming, as the docs say, that sqlite is compiled appropriately, and also a recent-enough version to offer foreign key constraint enforcement, of course).
You can also turn on Foreign Key support via embedding in connectionstring:
foreign keys=TrueExample:
"Data Source={DatabaseFullFilePath};Version=3;foreign keys=True;datetimeformat=CurrentCulture"