From softfork 22.8888, custom_json tx has been unstable. I know that the code is not related to cumstom_json. 22.8888 only blocks some txs of some accounts.
My best guess of this instability is due to api server is too unstable (miss anyx.io), and custom_json is not part of consensus (as far as I've heard, where can I see this?).
@steemfinex started about 8 months ago. No problems until very recent. But two incidents (which I manually resolved) occur recently and this isn't something that i can resolve automatically, since api returns success, and I should believe when it's successful to avoid double transfer. All I can do is check and transfer manually, which incurs lots of time. If this continues, I may discontinue the service, of course, with notice and enough grace period.
Base on the nature of the problem, this can only occur for STEEM => STEEMP, which requires custom_json writing tx.
When you think you don't receive STEEMP which you should, leave a comment on @steemfinex or @blockchainstudio. Also, there is algorithm to check balance sanity, so usually it's automatically detected, so don't worry too much. But please be patient since it should be resolved manually.