Log messages for queue processing are unpredictably dropped

a) The TaskContext includes an instance of PEAR's "Log" class which points to the CiviCRM log file – as provided by CRM_Core_Error::createDebugLog(). The same "Log" instance can be used for multiple writes.
b) CRM_Core_Error::debug_log_message() calls CRM_Core_Error::createDebugLog() and opens the log, writes to it, and immediately closes it. Thus, each Log instance is used for a single write.

In some (but not all) use-cases, debug_log_message() fails to write messages to the log. This is most likely due to the differing open/close patterns.