Two old-month-old American Standard Champion after 6 weeks, began siphoning bowl water after a more than unusual amount of rainy days. We have a 40-year-old septic system, pumped out 2 years ago. The tank is in decent shape with an acceptable layer of crud at the bottom but one of the two drainpipes from the distribution box is plugged. During these rainy periods. we only do one load of laundry or run the dishwasher. We plan on trying to auger, power jet, or replace the problem section. Trying to hold back on a new system. We only do one load of laundry or run the dishwasher. Nothing goes in the toilet other than poo or urine, no garbage disposer, and nothing other than water down the sinks. We have had no backups.
Only my wife and myself in a 2000 SF ranch. We currently are doing a remodel on another bathroom and the toilet has been removed with a rag in the drain, a new 5x3 foot mud pan with Kerdi membrane liner, and conducted a successful 30-hour leak test with 2 inches of water and water drained completely in 15 seconds or so. Afterward, no siphoning of the toilet. Don't have any issues with other drains.

I realize that a bad or clogged vent, cracked toilet, or bad wax seal can be the cause, and I plan to take action on those issues if necessary. My main question is can a slow main drain caused by a problematic septic system cause the siphoning? Are there any tests that I can run while running faucets, filling the bathtub, and releasing water in any combination with flushing the toilet? I suspect the septic system is the problem but I cannot figure out how that can cause the siphoning. I just don't see the correlation. Maybe a good example or explanation is the old dipping of a straw in a glass of water and trapping the water with a finger covering the end of the straw.

Apologies for this lengthy novel, but I know the members like all the details.

Update. It was still siphoning 3 hours ago but now after flushing a load, it drained and filled the bowl to the normal level.

I know that a partially clogged toilet or very slow draining drain can siphon a fixture as well as very heavy winds, does the toilet have a 2 flapper system

Just a standard one flapper. It's a 2-month-old American Standard Champion, which I have found to be a very good toilet, in my estimation. No clogs in all 5 of these units I've had in 2 homes in the last 10 plus years.
Pretty sure the slow draining in the septic system is the issue because the siphoning occurs only during extended rainy periods.