Select failed from V$LOGMNR_CONTENTS when called from a procedure
V$LOGMNR_CONTENTS

V$LOGMNR_CONTENTS contains log history information. To query this
view, you must have the LOGMINING privilege.

The LOGMINING privilege was granted to DBA role. When running an anonymous block, all your privileges granted through a role are effective. When running a stored procedure defined with the default definers rights option, privileges granted through a role are ignored. The LOGMINING privilege should be granted directly to your user:
grant logmining to c##myadmin;