I am a little late noticing maybe, but: Before we made the default framework authoritative for Koha-to-MARC mappings and removed the option to map in the framework configuratoin pages, you could map uppercase subfields to database fields with no issue. But now forced to use the Koha-to-MARC mappings page, this is no longer possible. There will be no error, just the mapping is not daved. Tested with 952 C trying to map to items.permanent_location.
There is nothing in the logs afaikt.
I was mistaken: the problem appears when the field is only in some frameworks, but not in the default framework. We should not fail silently.