Training load gone

Hello,

I'm having problems with the Training Load graph.
Sometimes activities disappear.
For example, I did a significant activity on April 13, and after synchronizing, the Training Load was correct and up-to-date.
A few days later, after a recovery activity, the Training Load was completely different, and the peak activity of April 13 had disappeared.

Can you explain why?

I always use the same two devices: a Forerunner 255 and an Edge 540 Solar.

Thank you.