The answer actually isn't known, but there's multiple possibilities due to several concepts!!!!!!

Crippling alcoholic, they often are when they are in a bad mood or something. Especially depression. I know when they first left him they had a miscarriage. But I have later concepts. Though I know they never truly would move on, during the 1910s Maveth was actually paying debts and doing work for Mr. Graves until Aleksandr became a cultist. Because at first the only reason Mr. Graves didn't kill or torture Maveth for denying debts is because she was with his dealer. He only didn't do it at all after she had half his family against him.

Another concept was that A COUPLE YEARS AFTER MISCARRIAGE AND DEPRESSION they noticed the Arachnid gaining consciousness more (which he did around 1909-1910) and saw it as time to sacrifice Maveth to fulfill the ascension of Icarus. So the members killed them. Maveth is STILL alive now because they were resserected in the late-mid 1910s or early 20s once the Arachnid began to become EXTREMELY sickly after their death. Though they might have had a different death completely and gotten resserected anywho ykyk.

There's also a concept that Maveth went to France/or just Europe to study the Corbeau magic?? There may be some cultists in France. I was gonna do something with that. Idk. There was some others too but I'll be honest I JUST pulled most of these out of my booty hole...