Like @Darren_Murphy said, I can almost guarantee 100% that the column is indeed marked as user specific since it’s attached to a google sheet. That is one quirk with the Multiple Files/Images column types. They do not sync to external data sources and you are stuck with them being user specific, meaning UserA can fill it with files, but it will be empty for UserB and vice versa. This does not happen with Glide Tables.
Typically, user specific columns are not able to be used in an Add Form or Add Row because it’s generally thought that a row would be created for multiple users, and later other users would fill the user specific columns with data specific to only them. There is also the fact that user specific columns are heavily reliant on on Row ID’s, but a row id is not created until AFTER a row is added, so essentially if it was possible to write to user specific columns at the time a row is added, they would most certainly break because there would be no row id to link the user specific values to the parent row. The values would just be lost in the ether.
Think of user specific columns like this. Behind the scenes, user specific data lives in its own separate hidden table with a type of relation/lookup in the parent table that links it to the child row in that hidden table, using user email and RowID to create that link. In reality, user specific column data does not live in the same table where the user specific columns are located. If there is no way to establish that link when the parent row is created, then there is no point in allowing people to fill user specific columns at that time.
I truly wish this is something that glide would fix. What they should be doing is allowing data from a Multiple Files/Images column to be written to an external data source, but maybe they store it as JSON or a delimited string which is just converted to an array on the Glide side. We should really have a true basic array type column that stores as JSON or a delimited steing, but displays as an array. I think it’s easily doable, but they would have to rethink how they are doing it now. There really isn’t a need for separate Multiple File/Images type columns. It should just be a Basic Array type instead in my opinion.