Basement Toilet Troubles

Got asked a question about a plumbing problem, maybe you can help

House has an older septic system. When it rains (as it sometimes does out here in western Washington) the water in the daylight basement toilet bowl drains, when the upstairs bathtub is drained. I understand it is probably a vent problem.

My question is could it be rain water filling up the vent pipe, or ???

What's a daylight basement toilet bowl?

There's a siphon happening in the shared waste line. Sounds like a partial obstruction just downstream of the lower fixture.