'Show heartbeats' output may indicate heartbeats from modules to CPM are being lost even if heartbeats are not currently being lost. The fix is available in XOS 9.7.1 or later.

'Show heartbeat' provides Link quality details of the heartbeats that are used for health check of the modules as described in article 4871. The link quality percentage is based on 16 heartbeats in the span of 4 seconds. However, in XOS versions 9.6.x and later, a benign issue is identified where the heartbeats received by the CPMs are not shown as 100% even though the actual link quality is 100%.
This discrepancy occurs due to a lingering affect of an actual event where the link quality dropped below 100%.

Open a case with Bluecoat/Crossbeam support and provide the following information to validate if the heartbeats are actually being lost or if it's due to the benign issue mentioned above.
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1. Capture output of cat /proc/heartbeat from CP1 i.e. CPM on slot 13.
2. run following command 5 times
/crossbeam/bin/cbs_hbquery -D 0
3. Repeat steps 1 and 2 followed by one more snapshot of /proc/hearbeat.
4. Capture output of cat /proc/heartbeat from CP2 i.e. CPM on slot 14.
5. run following command 5 times
/crossbeam/bin/cbs_hbquery -D 1
6. Repeat steps 4 and 5 followed by one more snapshot of /proc/hearbeat.
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If there is only 1 CPM in the chassis then run steps 1-3 or 4-6 depending on the slot in which the CPM is present.  

A patch is available for XOS 9.6.4. The fix is implemented in XOS 9.7.1 or later, and it is also tentatively scheduled to be implemented in XOS 9.6.8.