Shared Pool Issues possibly caused by In-Memory

Our test instance we had some issues with the shared pool. Analysis shows that the issue started with lots of entries similar to the following: ORA-04031: unable to allocate 2875560 bytes of shared memory ("shared pool","unknown object","sga heap(3,0)","ktmc_dlm_lck") ORA-04031: unable to allocate 2872832 bytes of shared memory ("shared pool","unknown object","sga heap(1,0)","ktmc_dlm_lck") ORA-04031: unable to allocate 2872832 bytes of shared memory ("shared pool","unknown object","sga heap(1,0)","ktmc_dlm_lck") ORA-04031: unable to allocate 2690496 bytes of shared memory ("shared pool","unknown object","sga heap(1,0)","ktmc_dlm_lck") ORA-04031: unable to allocate 2917536 bytes of shared memory ("shared pool","unknown object","sga heap(4,0)","ktmc_dlm_lck")

After an hour or so, the problem seemed to the 'spread' to other areas of the shared pool, messages such as the following: ORA-04031: unable to allocate 40 bytes of shared memory ("shared pool","unknown object","KGLH0^405b049a","kglHeapInitialize:temp") ORA-04031: unable to allocate 40 bytes of shared memory ("shared pool","select /*+ index(idl_char$
i...","KGLH0^5c4b4dd2","kglHeapInitialize:temp") ORA-04031: unable to allocate 480 bytes of shared memory ("shared pool","UPDATE SYS.AQ$_SCHEDULES
SET...","KGLH0^16fdfe24","kkslpkp:kksclitval") ORA-04031: unable to allocate 720 bytes of shared memory ("shared pool","unknown object","KGLH0^206fd69a","kkscs")

Have been doing some research, it appears ktmc relates to "IM Txn Block". https://dmitryremizov.wordpress.com/2015/10/19/a-new-im-related-oracles-diagnostic-events/ This makes sense as we have been researching InMemory option. I am guessing the ktmc_dlm_lock allocation is some kind of mechanism to ensure read-consistency.
My guess at the moment are that the size of the memory requested for this purpose, around 3MB, is more than shared pool is designed to handle. (Can anybody weigh in if I am wrong here? What about optimizer parsing huge chunk of SQL, would that require large chunk of memory like this?)

My guess is that the fact that this memory cannot be allocated causes existing allocated chunks to be moved around, subsequently causing issues elsewhere.
Am going to have to re-read the section in Jonathan Lewis's excellent "Oracle Core" book to see if this bears any relationship to reality.

In the mean-time, anybody want to chime in with thoughts/observations?

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