About restart training

[unrue]

I resumed training using Yolov5, v8 and now v9 and I noted the same weird behaviour. When the training restart, the mAP50-95 showed is less than last checkpoint. Example: 500 epochs total, restart from 280, last epoch mAP50-95 =0.34 and now the start value is 0.30.

Why, in general, there is a initial loss of mAP50-95 value when the training restart? Thanks.