I have had this problem for a number of months now, and have monitored closely the discussions online. Let us be honest here: there is NO solution to this problem to date. I have followed Dell's advice to update everything; BIOS, drivers, ran hardware diagnostics (all passed), the last time being two days ago. Surely the brightness control does come back after all this, only to fail again *apparently* after the machine goes to sleep more than once.
For those who work sometimes into the night, such control is crucial. If the latest update clearly does not work (to anyone), then Dell released an update that was not tested properly and should stop telling customers to update their drivers, BIOS, etc. Worse yet, you can find online recent advice from Dell to "roll back" the drivers. What is this? It equates to saying "here is the latest, it's been automatically installed on your machine, but don't use it".
Dell, please solve the problem. As of now, your product does not work and is impacting the work of those who need is as their main tool. It equates to being a hammer without a handle.