[fauigerzigerk]

It seems rather plausible to me what the article has to say on this exact question:

After that 2020 article, Amazon curbed a yearslong practice in which its own brands were given a boost in the search results on its site in special placements—the kind of edge other sellers could only gain by buying ads—according to the people familiar with the recent changes. That change caused many of Amazon’s brands to be buried in search results, making it harder for items to sell. The cost of warehousing all that inventory was significant for Amazon, making it a target during the cost cutting.

Doesn't mean they stopped the practice completely, but apparently it was enough to make most of their own brands unprofitable.