Plumbing – Could this trap be siphoning

I would say the overflow or whatever is on the sill plate would be more likely to cause a problem it looks like you have enough area for a trap but the angle could be deceiving. If there is enough water to trap the gas the next thing to look at that may be pulling water out of a trap is a plugged vent. I have seen plugged vents pull water out of several traps when a toilet was flushed.

Added per op photo: Ok wow the leak looks to be from a no hub or fernco coupler. That may be the source of the smell, in a case like this I like to run some bleach water prior to working on it even in my own home. 2 reason it flushes nastys down ant if you can smell the bleach the fernco is leaking. If this is leaking it would be best to purchase a new one I like the metal reenforced like you have they keep the rubber from bulging. These are a rubber sleeve with pipe clamps at top and bottom, the plastic pipe on top and cast below both fit and this is the seal. Tips use a wire brush on the cast to clean it up. Wipe down the plastic pipe. I put a coating of silicone calking in a few of the ribs inside the rubber this helps to seal especially on the rusty cast not a lot barely fill 2-3 ribs or a light film if no ribs. Get it in place and tighten the hose clamps. , I don’t think your trap is bad but when a toilet flushes if your vent above is not clear it may pull the water out but the design looks ok.