IXDP 20N60B IGBT reversed biased conductive
Not all IGBTs are capable of blocking reverse voltage.  If you need an IGBT for switching AC voltage for example you have to use one that is specifically rated for that application.

From this app note:

..the IGBT consists of a PNP driven by an N-Channel MOSFET
in a pseudo-Darlington configuration. The base region of the PNP is not brought out and the emitter-base P-N junction, spanning the entire extension of the wafer cannot be terminated nor passivated. This influences the turn-off and reverse blocking behavior of the IGBT, as will be explained later. The breakdown voltage of this junction is about 10 to 50V and is shown in the IGBT symbol as an unconnected terminal (Figure 2). For this reason IGBTs have an undefined reverse conduction characteristic...

However, IGBTs with defined reverse blocking capability do exist.  This paper explains how they work:

The primary difference in structure between the Fuji RB-IGBT &amp; a conventional IGBT is that the former has deep junction isolation structure that limits carrier generation thereby providing the needed (higher) reverse blocking
capability.