Yes, clean and lube my RD before most rides. I scrub/wash it and spray either teflon or silicone with teflon all over the der and wipe it off (in theory neither of those leave a sticky grease behind) and then put it through a few cycles. Same with the front. That specific instance above in the picture was after 4 straight days of dirt and dust but the trouble didn't start until the 5th day when it rained. I stopped and put some white lightning I had on the der eventually and I can't say if it helped or not but I was not in a position to actually clean it. The day before that trip, I replaced the end piece of housing that goes to the rear der knowing this happens to me
It is typical, any day in the dirt with lot of splashing dirt will eventually cause the problem. No mud/dirt, no problem. It is mostly on the upshifts when it happens. When it is clean or dry, I have no problems. I have about 8K miles on that Deore and a lot on trails/dirt/gravel, maybe it's getting a little old? I have a Tiagra der I could throw on but I don't like that compared to the Deore.
As for trying the cable liner. From the frame boss up to the upper frame boss effectively protecting the run down the chainstay? I think that might help. The housings I have are LEX-SL and have a liner already. I see this below but a comment in the reviews mentions that it specifically does not fit over the Jagwire sealed ferrules I have laying around. hmmm.. Maybe that is wrong. Any suggestion on cable liner and/or ferrules? I am willing to try even if that is not the root cause. It certainly won't hurt and dirt will almost always be in my route.
https://www.jensonusa.com/Jagwire-Black-Housing-Liner