You will probably see that the max_form_fields setting is set to 500. (250 rows with 2 form fields per row)
nmax_form_fields setting is set to 500 > no such field
Anyway... recordset is 900 irems total.
Multiple record insert > i only inserting 2 items out of the 900.
As long as item number is below the items number 250, all selected will be inserted.
BUT if i insert items 249 and i.e. 254. only one , the first one is inserted.