Your last sentence opens another idea - if this is happening in a subfolder, is the sample type also defined in the top level project and/or the /Shared project? If so, those settings may be different there. I can't reproduce exactly what you're reporting, but do some unusual show/hide behavior where those settings are sometimes inherited and sometimes overwritten depending on the order in which things are changed, so check those other containers to see if you have that same name sample type set to be always showing that field on insert.

If that's not it then re-creating new ones may be the only choice.

--Molly