UCC28056: Input AC current distortion

Hello Samo, 

Based on your schematic diagram, I can readily explain the leading 90° phase shift:
Output power = 30W, Input power = ~32W (estimated) at 230Vrms.  If PF =1, Input current = 32W/230V = 0.13913Arms.  Effective ideal resistance = 230V/0.13913A = 1653R.

Your schematic shows EMI filter X-caps C1 + C2 = 2 X 4.7uF, plus C6 and C18 add up to a total of 9.57uF across the AC line at 230Vrms, 50Hz. 
Reactance of this capacitance = 1/(2*pi*50Hz*9.57uF) = 333R.  This impedance is 1/5th of the effective resistance of your PFC converter, so 5 times as much reactive current flows into the EMI filter at 90° as does resistive current at 0°.    

Because of the distortion, it is difficult to see that the phase shift is not fully 90°.  If you maintain all the same test conditions and simply remove C2, you should see the phase shift reduce by nearly half to maybe ~50° or so.  Remove C1 as well, and the phase shift will be close to 0°.
Basically, you have far too much X-cap in your EMI filter.   

I cannot account for the distortion content of the current waveform.  
Some of it may be undamped ringing between the X-caps and the AC line inductance or filter inductance, but that is only a guess.   
Much of the step in current  probably comes from C18 (0.1uF) after the diode bridge.  This value is usually used for PFC at 100W or more.  
Near the zero-crossings, the inductor current is very low and cannot discharge C18 as fast as the line voltage falls, so C18 supplies all the inductor current and AC input current drops to 0A. 
I suggest to reduce C18 to 0.033uF or 0.022uF to reduce the step in current. 

I suggest to significantly reduce your X-capacitance first, to achieve your PF target for 30W at 230Vac operation.  Then follow-up any remaining distortion to track down and mitigate its source to improve THDi.

Regards,
Ulrich