I have a job that the Sony ES (3100ES) and it won't go into multi channel stereo when the Dolby Digital in on in the DirecTV 4K receiver. It works if I go into the DTV receiver and turn DD off.
I've never had this issue before, but just started to use the Sony ES line (used Yamaha Aventage before). I haven't had a chance to try it on another ES yet.
Any ideas? or is this just the way the ES processes DD?

This is my experience with the ES receivers, as well. I vaguely recall looking into this and seeing that the manual confirms multichannel stereo is not an option with a DD signal.

Back in the early 2000s I used to love Sony AVRs, installed a ton and even worked for a “Sony Only” store. The fact that the AVRs wouldn’t do all channel stereo in any shape or form always bugged me so eventually I stopped using them. Guess I still have good reason to stay away :(. IDK what their reasoning could possibly be for doing that. Shame because I loved the build quality and controllability. Also always bugged me that there were no rear control ports.

Sony was one of the pioneers of rear control ports as early as I can remember with their S1 and R1 control ports. one of the improvements on there CDP-102 was the addition of Rear control ports in fact....

If you want to run 5ch Stereo all the time, cant you just put the source component in PCM mode?

The S1 and R1 from what I understood was only good for other Sony gear to talk through. You would hit play on the CD player and it would send the CD input command into the AVR, and that was about it. There was no way to convert and send IR or RS232 into them. Yeah, I suppose on their newer ones you could do PCM from the source but then there’s no option to go back to 7/5.1, selectable from your control system, like on seemingly every other AVR in the world.

This was a LONG time ago, but I'm pretty sure the S1 input was a standard IR input for the device. It just was not expecting a carrier (much like the Marantz RC5 [RCA] connections at the time). Xantech made adapters for both during those years.
I had an S1 input on my Sony RPTV, that I used the Xantech piece on for years. Think I used it with a Pronto base station at the time. Early 90's I think.

Modern Sony receivers have standard IR inputs & outputs on them, starting with the STR-DN1080. And RS232 & IR for all the ES line. Not sure at what models IP control start.