There are several possibilities:

(a) The PMOS is used as a MOS capacitor. The gate is connected in this case to some other point.

(b) Only the diode between the diffusion and bulk is of interest. In this case, one may even short the gate to the drain/source.

(c) The PMOS is just a dummy device which could be added in parallel to existing PMOS current mirror branches.

(d) Someone made a cascode current source and later figured out they didn't need the cascode device or couldn't use for swing reasons, and shorted the drain/source connection with a metal mask change.

(e) A trivial attempt to confound people doing reverse engineering

Regards,

Vivek