If the value exceeds setting the indexer will not index that document and the entry is logged in indexer.log
The formula to calculate max_array_seckey_size is as follows:
If the index definition is - create index idx on bucket( a, distinct (array (x.y) for x in b end) , c )
then max_array_seckey_size = ( size ( a ) + size ( y ) + size ( c ) ) * (num of elements in b array)
even though it replies OK for POSTing to that endpoint.
Have you verified that that endpoint works 4.5.1-2844 Community version?
What might be another approach for finding this setting? Also, your second comment seems to imply that this setting is different for every index. Is that the case? It seems odd, given that we set it by POSTing to a general-purpose settings endpoint that is not index-specific.