Yes, it happened in the past.
Yes, I'm aware of a few incidents in the past where this has occurred (I think the Soviet shootdown of the Korean airline plane was preceded by the Soviet jet firing rounds as warning), and apparently the video shows it is SOP for the JASDF, but I'm wondering whether it is SOP for all other nations air forces as well to use tracer rounds as signal warning.
In particular, how normal is it for fighter aircraft to do interceptions by getting literally behind them on their six and firing tracers.
It seems like PLAAF sent some rookies who shouldn't be there (or acted on their own accord) in the first place so they escalated the situation (because JSDF can't open fire first and everyone knows it - aside from them it seems)...
At least it looks like that. That's probably how it went. Both countries got that in a diplomatic way away from the press to keep it quiet until retired JASDF general mentioned it in his article.
Come on, that's stretching it. If you're in a fighter jet and the other side tries to get on your six, then the pilots need to assess the situation within the security climate, I think the least responsible thing for a fighter pilot to do is to allow the opponent's fighter aircraft to get immediately behind you, even if you believe they are "only" going to fire tracer rounds.
If the F-15s really did light up their fire control radar then that is even more reason for the Su-30s to not allow the F-15s to get behind them.
If anything it seems like the JASDF got a bit eager and tried to get one over the Chinese Su-30s by trying to get on their six, but ended up getting done over themselves in response and had to let out flares to signal their disengagement.