This is still happening today. Three times so far.
I generate the Access Token in YOUR web interface: at https://apps.twitter.com/app/5401856/keys
I then use those to send DM from my application to subscribers of our service. They are following my account and DO receive the DMs while the access token is listed in your web interface at https://apps.twitter.com/app/5401856/keys
Randomly, although three times today, the credentials disappear from https://apps.twitter.com/app/5401856/keys and thus break my application from sending DMs
Once I generate a new Access Token and configure my app to use it, the DM function works as expected and I can authenticate and send DMs fine from my SaaS.
I am not using a “sign in with twitter flow” - this is a SaaS application that is sending DMs on behalf of the twitter account @securejoomla which is MY account for this service.