MySQL Still purging binary logs and ignoring expire_logs_days
(Not an answer, but too big for a comment.)

Flipping over to a new binlog happens two ways:


passing max_binlog_size, or
FLUSH LOGS


But neither should cause the deletion of the previous binlog without PURGE or expires.

Sounds like FLUSH is happening, probably from the 'nightly backup'.

Please provide the details of your "nightly backup"; the answer is likely to be buried in there somewhere.