Total speculation:
I have always thought that AC may have murdered JR (not that it's of any consequence at this point), and that if he did, the infamous argument he had with Lori may have been related to the murder.
Although the autopsy listed something like atherosclerosis as the cause of death, I think somehow it just became easier to say JR died of a heart attack. JR did have heart disease and high blood pressure, but as I understood things, the ME could not administer the specific test that they would typically use in an ER (to confirm that a heart attack has occurred), as it was post mortem (or maybe too long post mortem)
AC clearly hated JR. Not only did he taser JR and threaten to kill him, but he was supposedly still trying to arrange for JR to be harmed even while AC was in prison.
JR's custody battle with Lori over Tylee lasted almost 10 yrs. That's a long time for Lori to brainwash AC into thinking JR was the devil himself. Apparently, after JR's death, Lori never even notified JR's sister (who reportedly had always gotten along fine with Lori). I think JR's sister said she didn't even learn about JR's death until almost 5 wks after it happened (when someone called another Ryan relative about claiming the body).
My theory is that AC committed the crime without first discussing it with Lori, and then later surprised her with by admitting the crime (perhaps thinking she'd be pleased, but she instead went ballistic, idk). Again, total speculation. The timing also seemed to fit (JR death in Dec '18, argument in early '19). Jmo