Tracked data changes afterwards

I have included Matomo in two different websites and since the beginning of July, we’ve had problems with the data changing afterwards. We’ve noticed this in the evaluations of April, May and June. For example, the number of visits in April was indicated as 26.646 when I first created the evaluation in early May – and today it is at only 1.069 visits. I have attached two screenshots of our April Analysis: One is from May and the other one I took this morning.

Do you have an explanation why the data would change afterwards? And which numbers are correct?