IPN in sandbox not working for type mp_signup

[Nanosystems]

Hi, the sandbox is not sending any IPN notification with type mp_signup

i receive notification of other types like web_accept and merch_pmt, but mp_signup is not working at all

I checked at the IPN History but i can only see web_accept and merch_pmt notifications. I also waited 24 hours but my mp_signup still don't appears.

this same environment was working when i worked with it the last time, about a couple years ago

what i can try?
here a screenshot of my IPN history, they are all webaccept and merch_pmt