Sessions has never made sense to me.
First, I refuse to believe that Sessions and Trump didn't talk out strategy and discuss what he would do in regards to the Russia investigation and recusing himself. And as it turned out, he didn't need to be involved anyway, Mueller effectively cleared Trump himself.
But second, look at the work Sessions did on the border in his short time as AG. He locked the border down and made an insane number of human trafficking arrests. Other than recusing himself over Russia, Sessions checked off every box for Trump.
It's possible that Sessions simply caved and went back to his establishment roots and didn't want to fight.
Mueller didn't clear Trump he was going to recommend impeachment on bogus crap (that 100 pages + of nonsense he put together) claiming obstruction of justice for mean tweets etc. The swamp creature Barr put a stop to the actual official impeachment recommendation probably to try and bolster himself with Reps while he later covered for the crooks. If Rosenstein was still making the final call then we would have had another joke impeachment attempt almost guaranteed.