[Koha] Authorities lost during upgrade to 3.6.1

[Paul]

Hoping someone can help, please.

We recently upgraded from 3.2 to 3.6.1 and nearly everything went well 
during import of ~15,000 biblios, but something has happened to the 
"authorities" -- only newly catalogued (i.e. using 3.6.1) authorities 
appear in the staff search from 
<http://koha-admin/cgi-bin/koha/authorities/authorities-home.pl>.

All the others are still there:  exportauth.pl finds all PERSO_NAME, 
TOPIC_TERM etc as expected, e.g.:

00389nz  a2200133o 
45000010002000000030007000020050017000090080039000260400019000651000017000846670040001016700099001419420015002408OPIACS20110310122059.0110310|||a|||||| 
| |||     d  aOPIACScOPIACS  aAbbass, D.K.  aMachine generated 
authority record.  aWork cat.: (OPIACS)3: Abbass, D.K., Radeau Land 
Tortoise, North America's oldest warship, 1993  aPERSO_NAME

but Koha no longer appears to "cross reference" them as far as staff is 
concerned.  For example, when a cataloguer attempts to enter another title 
by Abbass, D.K. (the "authority" I quote above), she cannot find it in the 
drop-down for 100a

I'm sure there's a simple fix, but at the moment I'm tearing my hair out -- 
thanks in advance.

Paul