I like to start breathing using my own rythm and then after a while the intervals widen naturally and I could then synchronize my breathing with one of the well known patterns but usuallly I don't. If I try to start with e.g. 4-7-8 I have the same problem of going into panic mode and then I am not relaxed at all and heart pulses go up instead of going down.
for me the out of sync happens during the transition between cycles. I think the problem is the visualization in this 4-7-8 app. you don't know when the exhale is going to end, you are almost at the end of exhale and inhale starts immediately. .There should be a small gap of 1 between the cycles 4-7-8-1-4-7-8-1 ... or the animation should indicate when the exhale is going to end.