"Sudden Mantis Death Syndrome" is a common thing, unfortunately. I had this problem a lot with Zebratas. I did find that using my Sanitizing Spray (I sell it on the website, it is just colloidal silver), helped them to pull through, so my theory is that it is a bacterial infection. Colloidal silver creates an environment in which the bacteria cannot grow (or reproduce), so it apparently prevented the infection from getting any worse and the remaining bacteria died out.
I don't believe that mantids are prone to viruses, but even so, silver kills viruses too!
Not trying to "toot my own horn", just offering that up for helpful information.
Zebratas were the first species that I commonly encountered this problem with, so that is why I tested the spray on them. Out of 3 that were "sick", only one died after treatment (it may have been too far gone already.) I used the spray on them once a day or every other day.