Some spottings have wrong reported Power value?

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Some spottings have wrong reported Power value?

I have a question regarding some odd entries in the database. Yesterday, I started my WSPR beacon in the morning, with power set to +33 dBm (2W). After a few minutes, I decided to change the power to +30 dBm (1W) to be slightly closer to what my SWR/Power meter indicated. Almost every spotter picked up the new power setting in the message, but there are a few entries in the database that look odd.

At 2018-05-07 0714Z I used +33 dBm, and then from 0734Z I used +30 dBm. Surprisingly, at 0834Z (i.e. an hour later), two different stations reported my station as being +33 dBm.

At first, I thought that this could have been an issue with UTC vs local time, but two other spottings (at 1104Z and 1304Z) were also reported as +33 dBm rather than the correct +30 dBm.

What is the most likely explanation for this phenomenon? Is the database randomly broken? Is my beacon transmitter (a QRP Labs QCX) buggy, sending out a bad message? (As far as I understood, the checksum involved in the wspr message would make such a mutation of the message unlikely.) Or is this a known bug in the reporting software?