It's possible some of the tables showing STATS_TIME "-" in syscat.tables catalog table even when the auto-runstats in the database is tunred ON. The main reason might be, if a table is never accessed, then auto-runstats will not action on that table, even if the table has never had runstats done. This is by-design. This could be applicable for system tables as well as user tables. Regarding system catalog tables there might be differences of observations from...
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Tags: 
auto-runstats
db2
stats_time