it's not a
major bug, check 1:08
aerobatic routines are made by exasperated manoeuvres normally, but you can easily get a controlled spin going with a high performance aerobatic aeroplane. I can't tell from the video whether you had any throttle in, but if you did then it's quite plausible cos you're keeping the rudder in a strong slipstream, if not then yes, that's a bit of a bug. The sink rate is probably a bit too generous (the stall speed for these beasties is around 57kts), but bear in mind that you have quite a generous symmetrical profile wing surface and control surfaces and rudders in particular are VERY efficient in this family of aerobatic aircraft. The Su-26 can really hang from the propeller and hover there, so yes, extreme aerobatics are achievable.
I haven't tried the new plane yet, but I'd be more concerned about stuff torque effects and gyroscopic precession. Do you have any of that?