If only a stump is left there should be no molting issues. Depending how soon it is to the next molt there may not be enough time to regenerate it until the following molt.
Even when damage is done during a previous molt, the best you can hope for is partial tiny leg from a regeneration molt. A second molt afterwards tends to make the leg functional again. Then after that any additional molts don't seem to improve the regenerated leg (it will be a function leg after the 2nd molt normally, however it will always be a bit smaller and easily identified).
Depending on how long it is into it's current presub instar stage it may wait until the sub-adult stage to do any regeneration, and if that is the case it will only have a partial tiny leg and likely nonfunctional. If it starts regenerating the leg before the next molt (sub-adult) it should return to a function leg when it reaches an adult.
I've had a few mantids with missing legs, especially as old/elder mantids (they often fall apart before dying), and with only one missing they will learn to move normally without it usually in a bit of time. I also had one mantid that only a single raptorial forearm/front leg it's entire life, and it learned to walk and grab prey without it (I named him Bandit, and you can
see photos here), he lived a long mantid life.