Repeated Disk Contamination
It's probably some kind of unfixable (given the lack of replacement parts) problem with the pistons. If you want you can take apart the caliper to confirm, using a blower attachment on an air compressor to pop the pistons out and inspect them. But what it comes down to is if your no-service-parts-availability caliper is leaking oil repeatedly, there's not a lot you can do but replace it. Luckily a new BR-M785 complete with finned pads is under $50US.