I got too much from the subscription
I started my subscription at 6:40 AM (JST) on February 10, just after the CORE service started, and continued without a single interruption.
I reinstalled the app a few weeks before May 8, so just to be safe, I went back to the scanner map screen at 10:34 AM (JST), right after I refreshed my subscription from the settings, and got my 4th subscription count and reward.
I also received my 5th subscription count and reward when I launched the scanner at 07:02 AM (JST) today, May 10.
It seems very likely that the 4th subscription count and reward on May 8 is a bug.