Apple Watch Ultra ECG random and false measurements

I have an unclear issue with Health app. I do running almost every day and each time I'm finished with it get ECG readings in the hear rate detailed list, among the regular Streaming but did not do any ECG measurements by myself. And those ECG are very low against regular readings. E.g Streaming is still 93 before and after ECG but ECG is like 54. And this pattern is executed almost after any excercise I do.

How and why watch does this by itself and then it does a false readings. If I to regualr ECG readings results are fine.

Besieds, have no health issues on that front.

Regards