This is really weird!
I created several custom fields to quotes (and to invoices)
In some cases (not sure exactly when this triggers), filling a custom field and hitting the ‘save’ button clears the content of other custom fields of that same record. If I exit the quote (or invoice) edit view and come back, the last added custom field has value but the values of the others custom fields are now blank. If I then reenter all values and hit save they persist.