Ok… some answers for you.
Firstly, turning on Ironing with the default Bambu studio settings (and Iron Top Layer Only) fixed all issues.
What is the rough spot’s orientation on the build plate? Front, back, left, right edge?
3 o’clock position. It’s on the uppermost layer (away from the build plate).
If you rotate the model, does the rough spot say in the same place or does it move?
Don’t know, only printed them this way round. 
What filament are you using? Temp? Build Plate Temp?
Matterhackers Build PLA.
Was the PEI Textured plate… brand-new from Bambu Labs (3rd or 4th print).
What infill are you using (grid?)?
Was using Grid, but have since changed to Hilbert Curve at 30%. The Grid was too little infill.
How many top layers?
It was 3, but changed it to 4. Have never had to do this before on any other print.
Fan settings?
All default…
Is the bottom of the print uniform in this area, or did it start to lift while the print was still underway?
Adhesion was perfect across each of the three prints (which were all printed on different days).
Shadow:
Looks like my problem:
Not really. Mine looks like bread dough or pastry dough that’s been bent a little too far!
P1PNoob:
Have you tried cleaning your textured PEI plate in soapy water? I was experiencing exactly the same problem, and cleaning the plate fixed it.
No. It’s a brand new plate and this is the only model that had the issue. The plate is exceptionally clean!
Kallvin:
A common reason I find is infill patterns causing collisions with the nozzle end, or something to that effect.
Could be. It disappeared when I used Hilbert Curve and increased infill to 30%, and used Ironing.
Thanks, all, for your help.
