Summary: It has to do with college education rates and Biden's view on bipartisanship
You have to understand that non white Americans as a whole are believe to have supported the Democratic Party with 50 percentage point margins at a minimum in presidential elections since 2008 based on exit polls and election results. It is also an oversimplification to say that it is normally supported by ... racial minorities. There is a decent 18 to 24% of non white voters which vote Republican.
These losses seem to be concentrated with non college educated voters. You have to understand that just because you are winning a group by 52% or so (which various sources put Biden's win at with non-White Americans) does not mean that every person in that group supports you. And, of course, there are independent voters involved too.
The article says that it is stronger with those without a college degree. Non white Americans have lower college education rates in general which might explain the swing since Republican policies are more appealing to lower social class individuals in recent years.
One reason is that Biden has a view of bi-partisanship. Approval rating does not necessarily mean how much people want to re-elect someone.
This image shows possible reason that this may be so. Though all major racial groups within the US have Democrats supporting finding common ground, that support is 12 points lower with non White Democrats. So it makes sense that Biden by pushing for bipartisanship may be alienating non-White Democrats and younger voters, the second of which tends to be more progressive than other groups of Democrats.