we have a replica set composed by a primary, a secondary and an arbiter. we deployed it with automation-agent from mongodb cloud manager.
everything is ok on the primary but we noticed that the free disk space on the secondary in constantly decreasing, checking the filesystem we discovered that the journal directory is the culprit.
we performed a full resync of the secondary, but after 24h the journal directory occupies 380Gb (we have 485Gb of data). see attachment of the free disk space graph over 24h (primary on the left, secondary on the right).

currently there are 3926 journal files in the journal directory. (see attachment)
the primary is not accumulating journal files and behaves as expected.