There are two often encountered scenarios that correspond to this problem:
1. The two switch contacts of the polarity selector are not closing/opening properly, due to mechanical distortion or misalignment.
2. If this is a pre-1986 U87, sometimes the two backplate halves, which should be insulated from each other through a 6µ washer, are screwed together too tightly and when they make contact, they mess up the patterns.
There is also the odd situation where someone screwed around with the mic's wiring and mis-connected the lead-outs between capsule and switch assembly.
In sum, this is quite easy to troubleshoot: with an ohm meter and the proper schematic you can trace continuity from each head pin to the proper destination of each wire.
Edit: your follow-up below, where you write that swapping heads does not change the problem makes my troubleshooting tips unlikely, at least for this mic.