If you are on most current version 1.27.108 it is because they updated the earnings to only show your current months estimated earnings not the total from before anymore. The previous BAT will still be paid out come August and are not lost, they just do not show.
“Current earnings this month (estimated)” do show rollover balance from previous month if any. See posts by LaurenWags here (closed issue #15005 in Release Notes)-
BATs most likely reduced now because they didn’t reduce after payouts this month. If reduction is equal to payout received then it’s fine.
If it reduced slightly more than payout received then likely due a bug mentioned in the post linked by @HighPriestess42