A primary goal of Europe’s recent “stress” tests of its banks was to illuminate their holdings of potentially risky government-issued debt. But that clarity has been fleeting. Regulators across the European Union conducted the stress tests of 91 banks last month, hoping to dispel investor anxiety about the health of the continent’s banking system. In addition to gauging the banks’ abilities to withstand an economic downturn, the tests also required banks to show the amounts of sovereign debt—debt issued by national governments—that they were holding as of March 31. Now, some banks aren’t providing updated sovereign-debt details.