Hello,
There could be a number of reasons as to why this happened, but a bug is not likely. Here are a few possibilities.

1) RNG. Critical hits factor hugely into your overall damage. If your first run saw you get very lucky and you got a lot of critical hits, you might be having trouble bypassing that score because you just got lucky the first time.

2) You accidentally changed your equipment setup and haven't realized it. This happens sometimes. I would suggest checking your equipment/tools/links and making sure everything is set up properly. Also your skillbreaks, basically go over everything to make sure everything is set up properly.

3) The trials grow harder based upon the stats of the strongest person in your server. So if they are outpacing you in growth, then the trials will also be outpacing you in growth, though this isn't necessarily a 1:1 ratio.

4) There could well be other things that are causing an issue with your team performance. I don't know everything going on with your account.

As for the lag issue, some teams do lag more than others. If you're using SSP naruto, for example, he causes a lot of lag, and there's unfortunately little way to get around that. :(

Ultimately, if you feel that this is still a bug, you can submit a ticket to the CS platform and have them check it out for you, but I strongly suspect it is not. However, here is the link for you.