What I did was:
1) in iCash File Manager, click New and typed a name Test.iCash
2) in the Default Categories and Accounts dialog box, I selected "iCash File", and checked "Copy Balances" and pressed the "Yes" button
3) Chose one of my existing iCash files.
At this point it opens the new Test.iCash.
4) I went to enter a new transaction.
The Origin Account, Target Account and Payee/Issueing fields were all case sensitive, ie it wouldn't autocomplete anything that was in the incorrect case.
5) I closed down Test.iCash and restarted it again.
6) Went to enter a new transaction - all ok now, no longer case sensitive.
When I wrote the earlier post to report this, I must have not yet closed and reopened my iCash file. I did try to do File/Tools/Check Data but that made no difference. Simply closing and re-opening fixed it.
I have just replicated it again and got the same behaviour as described above.