Finally I received a weekly bonus less than a dollar

I have no deposit for 2 months now and I wagered less than $100 during the last 7 days. With the same wager stats as well as PnL stats (less $100 wager and $1 profit), I still received $5 in the previous weekly bonus, but last night the weekly bonus is decreased signficantly as I received $0.67 only

It seems that you are right that there must be a change in the formula to calculate the bonus.
They have the metric used in calculating for the bonus being offered, they are not just giving the bonus just like that, but there are criterias they ensure that they looked into before giving any bonus to gamblers, if we want more of this, then it may require us to have increased activities in gambling, make some deposits, wager and be active in playing games.