-U doesn't work with a single offset value

[seisman]

Description of the problem

$ gmt plot -R0/10/0/10 -JX4c -Baf -T -Uabc+o-54p -pdf map
plot [ERROR]: Option -U parsing failure. Correct syntax:

  -U[<label>][+c][+j<just>][+o<dx>[/<dy>]]
     Plot GMT Unix System Time stamp and optionally append a <label>. Optional modifiers:
       +c Use the command line as the label [ON].
       +j Set frame justification point [BL].
       +o Offset stamp by <dx>[/<dy>] [-54p/-54p].
plot [ERROR]: Offending option -Uabc+o-54p

Using -Uabc+o-54p/-54p works.

System information

• GMT version (gmt --version): 6.5.0 on 2022.11.21