sql replication error - record exists on Publisher but it's trying to delete/insert that on Subscriber, so Foreign Key/Primary Key violation

We have transactional one-way replication running, and suddenly today started getting this error :

The DELETE statement conflicted with the REFERENCE constraint "FK_BranchDetail_Branch".  
The conflict occurred in database "LocationDB", table "dbo.BranchDetail", column 'BranchNumber'.

Message: The row was not found at the Subscriber when applying the replicated command.

Violation of PRIMARY KEY constraint 'PK_Branch'. Cannot insert duplicate key in object  
'dbo.Branch'. The duplicate key value is (23456)

Disconnecting from Subscriber 'SQLDB03'

Publisher - SQLDB02.LocationDB
Subscriber - SQLDB03.LocationDB

Tables on both servers:
Branch (BranchNumber PrimaryKey)
BranchDetail (BranchNumber ForeignKey references previous table)

select * from SQLDB02.LocationDB.Branch -- contains : 23456,'Texas',...
select * from SQLDB03.LocationDB.Branch -- contains : 23456,'NULL',...

The problem is - the BranchNumber in question '23456' exists in all 4 tables (Publisher..Branch, Publisher..BranchDetail, Subscriber..Branch, Subscriber..BranchDetail).
Yet, when I ran a trace on Subscriber, I see repeated commands like:

exec [sp_MSdel_dboBranch] 23456 -- which throws FK violation
exec [sp_MSins_dboBranch] 23456,'NULL',... -- which throws PK violation

I'm guessing it's trying to Update the record on subscriber by doing a Delete + Insert. But it's unable to..

Users do not have access to modify Subscriber table. But they can modify Publisher table through UI, and have been doing so for long time without issue. There is also job that updates Publisher table once every night. We started getting this error around noon today.

Our last resort is to reinitialize subscription off-hours.
But any ideas what could have caused it and how to fix it?