Tankless Rheem RTE18X AND Delta MultiChoice Valve questions/problem

Hi. So I have the afore mentioned Rheem RTE18X and I also have the afore mentioned Delta Multi running the grey RP46074 cartridge. The tankless unit can be adjusted upto 140 degrees. My Delta cartridge keeps throttling down the temp to warm from the 120 I have it set to. I have tried 110 and 115 also with the same results. I can set it and I'm good everywhere else in the house. Is this just a bad choice for use with a tankless unit? Is there a good choice or choices? Thanks.