Devonthink forgets if you set the “Check Spelling As You Type” to True (in the Edit/Spelling menu). At first I assumed it was remembering it on a per-document basis – you might want to check the spelling for an essay you are working on, but not for a web page you’ve collected or a code listing – but it’s not doing that. It’s just forgetting it database-wide when you finish editing a document where you have set it to True.
It is not the behaviour exhibited by TextEdit, Apple Mail, etc. Is this deliberate?