You said "forum_tp_clean_read_records will be triggered periodically to clean all read records" but then "read records beyond the forum_oldpostdays will all be deleted", so I take it that it doesn't delete all of the read records but just the ones that are for posts older than the forum_oldpostdays setting?
This is a feature, to keep the "read posts" table in reasonable limits. Otherwise it just grows and grows. 1 000 posts and 1 000 users gives a theoretical of 1 000 000 rows in the "read posts" table.
As for the "changing read tracking from Yes to No deletes the read records", this could be changed, so changing the setting back to "Yes" would still remember the read posts.
Forum
Sometimes "read" posts come back as "unread"
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