Accounting Processor GL postings to "secondary" schema unreliable

the client is set up to post documents to 2 accounting schemes in parallel, primary schema USD and a local accounting schema.
Sys.Config CLIENT_ACCOUNTING is set to "Q" and the interval is 10 minutes. The accounting processor runs reliably every 10 minutes.
Accounting Processor=>Accounting Schema is blank so postings are made to all schemes.

Generally, the system behaves as expected and posts documents to both schemes. However, we often also find documents, which only get posted to primary schema USD and not to the local schema!
E.g. there are many MR documents posted correctly to both schemes but there are also some MR documents which only post to the USD schema.
The system does not issue any posting errors. The only way to resolve this problem is to display the "partially" posted document (eg MR document), push the "Posted" button, select the local schema and push the re-post-button. The fact that a cache reset is required prior to re-post could indicate a problem in cache array variable s_schema in MAcctSchema.getClientAcctSchema().