Vikash,
You can see what cas is doing with logout:
<!-- INFO Performing logout operations for [TGT-...]
[number] logout requests were processed
DEBUG ST, principal and URL -->
<Logger name="org.apereo.cas.logout.DefaultLogoutManager" level="info">
<Filters>
<ThresholdFilter level="INFO" onMatch="ACCEPT" onMismatch="NEUTRAL" />
<RegexFilter regex="Captured logout request.*" onMismatch="DENY" />
</Filters>
</Logger>
<!-- DEBUG preparing, processing and logout with URL and ST -->
<Logger name="org.apereo.cas.logout.slo" level="warn" />
<!-- DEBUG outbound and inbound headers and response as it is sent -->
<Logger name="org.apache.hc.client5.http.wire" level="warn" />
Best to check these in dev first.
There may be some OIDC specific logging you could try; I have not explored OIDC logging.
Ray