There are vacuum diaphragm operated valves on the transmission. Well, there used to be anyway - electronics may have replaced all of them now.
Occasionally those diaphragms would split, or sometimes leak around the edge, which would then allow trans fluid to be sucked into the engine intake through the vacuum line.
Lol, I'm a dinosaur mechanic (worked at the local Ford and Chevy dealers in the '70s). Some of the things I learned no longer apply, but many still do. I just don't always know which is which...
