I'm currently trying to figure out what is going on.

I fill a form from my PDF and then I need to use flatten in order to "save" the values and then download said PDF.

At the moment of using flatten there is an Unhandled Rejection, as follows

Unhandled Rejection (Error): Could not find page for PDFRef 439 0 R

which I understand means that once the field is flatten, the library will delete the field and uses the ref value of the field in order to do this, in this case, it looks for a field with a ref value of "439 0 R" but it does not exist, so I looked around to see what was said problem, using the following code

const fields = form.getFields()
fields.forEach(field => {
    const type = field.constructor.name
    const name = field.getName()
    const ref = field.ref
    const acroField = field.acroField
    console.log(`${type}: ${name} - ${ref}`)
  })

and found out that the ref values only go around 200-ish, about the same number of fields from the PDF.

PDFCheckBox: cb_1_1_a - 11 0 R 
PDFCheckBox: cb_1_1_b - 12 0 R
PDFCheckBox: cb_1_1_c - 13 0 R 
PDFCheckBox: cb_1_1_d - 14 0 R
PDFCheckBox: cb_1_1_e - 15 0 R 
PDFCheckBox: cb_1_1_f - 16 0 R 
PDFCheckBox: cb_1_1_g - 17 0 R 

[...]

PDFCheckBox: cb_52_4_s - 208 0 R
PDFCheckBox: cb_52_4_n - 209 0 R 
PDFTextField: tb_53_1 - 210 0 R 
PDFTextField: tb_53_2 - 211 0 R 
PDFTextField: tb_53_3 - 212 0 R
PDFTextField: tb_53_4 - 213 0 R
PDFTextField: tb_53_5 - 214 0 R 
PDFCheckBox: cb_1_8_b - 215 0 R
PDFCheckBox: cb_1_8_c - 216 0 R

the field "cb_1_8_c " is the last field of the form and has a ref value of "216 0 R"

So, I have no idea what is happening, what is causing the function flatten to use a ref value of "439 0 R" when none of the current fields have such.

The PDF I'm using was a flat document that I later add field forms using online tools.