Physical Reads Direct on Standby

Hi All,
We've got a Standby database which has the apply lag suddenly started to peak at 10 minutes overnight during peak processing times, whereas previously it was near zero.   As usual everyone claims "nothing has changed" ! The only OEM tracked metric that seems to have changed is "Physical Read Direct" which is now bursting at the same times as we are getting the apply lags, whereas before it was negligible.   Redo rates at these times as the same as before and other CPU, Disk and Memory metrics seem pretty much the same.   The destination is SYNC AFIRM and the transport lag is pretty much zero. Any suggestions on how to diagnose this?    I'm not really looking to fix it right now, just find out what has caused it to start happening (19.14). Thanks for any tips!Charlotte

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