There's a strange problem that all buses' theta is constantly changing

[MuaYoi]

I used the ieee39_full.xlsx to simulate and added an alter that the 'Ppf' of load 'PQ_1' is modified from 6 to 6.2 at time 5.0
From my own perspective, this change is not significant enough to destabilize the system, so the phase angle of the bus should fluctuate for a period of time before returning to stability.
My code is here:
ps = andes.load(ieee39,setup=False)
ps.TDS.config.tf = 40
ps.TDS.config.tstep = 0.005
ps.PQ.config.p2p = 1.0
ps.PQ.config.p2i = 0
ps.PQ.config.p2z = 0
ps.PQ.config.q2q = 1.0
ps.PQ.config.q2i = 0
ps.PQ.config.q2z = 0
ps.add('Alter',dict(model='PQ',dev='PQ_1', src='Ppf',t=5.0,method='+',amount=0.2))
ps.setup()
ps.PFlow.run()
ps.TDS.run()
The result confused me, because the voltage curve met my expections, like the second figure but the theta curve is abnormal like the first figure.

[MuaYoi]

Let me add a picture, which shows the output curves of all synchronous generators.

[jinningwang]

Check this, #335