Since the maintenance upgrade/update on 31 May, all new incoming tickets are missing CustomerIDs.
See https://bugs.otrs.org/show_bug.cgi?id=10691#c15, in short - as far as I can see -: They removed the automatic assignment of CustomerIDs for new tickets in 5.0.16, then re-introduced the functionality in 5.0.17, but ever since then it needs to be specifically enabled through PostMaster::NewTicket::AutoAssignCustomerIDForUnknownCustomers. We should probably do that, since we don't assign CustomerIDs manually anyway.
I usually use the People → Customer to quickly find duplicate tickets, or old tickets by the same customer, which is not possible without an assigned customer ID, so would also recommend turning that thing on
From the changelog it looks like this new config item was added.
The related bug is https://bugs.otrs.org/show_bug.cgi?id=10691
2017-02-08 Follow-up fix for bug#10691: add new config setting PostMaster::NewTicket::AutoAssignCustomerIDForUnknownCustomers to control creation of tickets with unknown customers.
However the Default is Yes (per the upstream bug above). Confirmed via the admin interface, however it does not seem to work