It would be nice to have predictability.
I am not aware of any issue with capital versus not
I am aware that preceding and trailing blank spaces can cause issues.
but help us understand by pointing out to us exactly WHERE you are referring to - as to your comments.
as I almost get the impression we may be mis-understanding your comments. Please help us understand the context.
One additional thought for your issue of sometimes needing to use capital letters in the Person ID and other times not.
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I am just another 'lowly' User/Patron ...
Provided that you are referencing the 'FamilySearch Person Identifier' (PID), of the individuals/persons, in "Family Tree", of 'FamilySearch" ...
As far as I was aware ...
Over the YEARS, 'FamilySearch' had done considerable work, on the various pages/screen; where, a 'FamilySearch Person Identifier' (PID), is required to be input/entered.
That work included, that it DID NOT matter, whether or not, the "Letters" were either, "Upper" or "Lower", Case, during input/entry (Either, was ACCEPTABLE); and, also whether or not, the "▬" (ie. "Dash") was included, during input/entry (Either, included or not).
Are there any particular pages/screen; where, you are actually "Finding"/"Noticing", that there is a REQUIREMENT to input/enter a PID, specifically with, "Letters" in "Upper" Case?
The information given you by Brett is correct regarding the entering in of ID Numbers. Please post your question again of you are still having trouble entering them in.
Please review the Knowledge Article linked below: