I confirm same problem.
(EDIT) If my understanding is right, the tourism boost from flight was designed for improvement only. Thanh is district. Perhaps there is some clue on this, I guess....
(EDIT) I think it's CollectionType issue. Originally, EFFECT_ADJUST_DISTRICT_TOURISM_ADJACENCY_YIELD_MOFIFIER has COLLECTION_CITY_DISTRICTS as CollectionType.
But, Thanh has COLLECTION_OWNER as CollectionType.
In my test file, Tourism of Thanh district itself has no accumulation but top bar shows +2 Tourism per turn from Thanh.
If I have time, I'll try it can be fixed by changing CollectionType.
(EDIT) Just changing CollectionType doesn't work. FXS needs to check this.