Welcome @Fatcat to the forum!
I personally don’t have a EB150 unit but see from the manual it says the included AC wall charger is 200w/42v… you said you were pulling over 200w when it first started charging?? That is odd right off the bat but as the unit can accept 16-60v & 500w, I don’t see it being a problem or an issue that might hurt the unit.
As for the variances of inputted charging wattage with time, it could be the internal mppt charge controller regulating the input based off of the chemistry of the LG Li-Ion battery cells that the EB150 has?? This is an example of the ideal charging profile for those cells I pulled offline…
I can’t confirm or not that this is what’s going on with your unit specifically, but ideally the charging current will drop as the battery gets closer to “full” to protect the longevity of the cells.
Does it have the same sort of pattern when you’re charging via solar??
I’ll be curious as to what @BLUETTI says so keep us posted.