I applied 5 Aug and I got the IDV app invitation today.
The official deadline for August applicant is 14 September 2020, However, I decided to contact UKVCAS via Twitter DM. The answer I was expecting "Wait until 14 Sep.", But they said "I will look into this and get back in touch with you within 1 working day". After 1 day waiting, they clearly replied me "You should receive an invitation today".
As promised, IDV app and in-person appointments opened for me at 6pm today.
If someone is stack on IDV app invitation, It seems UKVCAS twitter is the right path to solve.
P.S. I assume My invitation delay was caused by I sent opt-out request. When I applied the application, opt-out process for Mar-July applicants was on going and the way was shared in this forum. I foolishly tried the request since I thought same process will be taken for Aug applicants and Faster request may make faster process. Of course nothing happened as August applicant. However, Probably (not 100% sure, but I believe) I was kicked off from IDV app invitation list by the request. so I sent "Opt-in me" to UKVCAS twitter.