I had a line that acted like that. finally had it all replaced after it leaked. how old is the home? the line was totally sluged up.......

it was copper and the interior was rotted.

had all PVC installed and its been years no leaks no backed up lines.

the downstream line might be bad leading to poor drainage and sluge collection.....

my main house drain lne has had root troubles for perhaps 15 years....

so a couple times a year dump a 25 pound bag of rocksalt in the basement washtub, mx hot water with a shovel to make a rich salt mixture, then everyone goes out for day.....

no root troubles anymore.....