Assuming that the poll is indicative of a trend (we don't know yet), Ukraine could be a factor to an extent considering his initial drop in support did coincide with Afghanistan and how that withdrawal took it's course.... I don't think that Americans disagreed with the withdrawal, but one of Biden's strengths going into 2020 was foreign policy and management of it and, to the extent that it impacted peoples views, fairly or on fairly, it was seen as contradictory to that strength.
His handling of Ukraine, by contrast, has been relatively well done (regardless of what Archie thinks) in terms of the actions taken, so it may have been only a matter of time until that started to reflect some in Biden's approval numbers.
The other part is COVID - things to seem to be normalizing a lot these days and that would probably be a benefit as well.
In any event, this Presidency so far can be divided into two parts - the part where Biden was sitting between 53-55% and the part where Biden is sitting between 41-43%.... during both parts, there was an assumption by many of inevitability, that those numbers could never change. People need to be open that external events do happen and opinions can change.