I'm going to +1 onto this, but it's not a new thing. I found a post on StackExchange finding this occurring after CS6, back in 2015. (https://graphicdesign.stackexchange.com/questions/36566/how-do-i-get-adobe-indesign-cs6-to-export-high-quality-pngs)
I'll recap their findings, and mine agree with it:
"It occurs when a graphic containing an alpha channel ("transparency" in the case of PNG, or the existence of any layer other than "background" for PSD) is displayed in InDesign at a size other than 100% (the image, not the boundary) and/or at a DPI/PPI different to that of the document, and is exported either as a PNG or PDF. It seems it switches to a different resampling algorythm for high quality output, but messes up somewhere giving this awful jaggy nonsense."
My experience mirrors their own. I've attached a screenshot comparing 2 pngs exported from InDesign. Each of them is nothing more than a placed PSD, and the only difference between the two is that the PSD has been flattened in one. The other only has 1 layer, and has no transparency, but since the only layer is not seen as a "background" layer, the file seems like it's viewed as having transparency and seems to cause the PNG to get ugly.
I'm going to +1 onto this, but it's not a new thing. I found a post on StackExchange finding this occurring after CS6, back in 2015. (https://graphicdesign.stackexchange.com/questions/36566/how-do-i-get-adobe-indesign-cs6-to-export-high-quality-pngs)
I'll recap their findings, and mine agree with it:
"It occurs when a graphic containing an alpha channel ("transparency" in the case of PNG, or the existence of any layer other than "background" for PSD) is displayed in InDesign at a size other than 100% (the image, not the boundary) and/or at a DPI/PPI different to that of the document, and is exported either as a PNG or PDF. It seems it switches to a different resampling algorythm for high quality output, but messes up somewhere giving this awful jaggy nonsense."
My experience mirrors their own. I've attached a screenshot comparing 2 pngs exported from InDesign. Each of them is nothing more than a placed PSD, and the only difference between the two is that the PSD has been flattened in one. The other only has 1 layer, and has no transparency, but since the only layer is not seen as a "background" layer, the file seems like it's viewed as having transparency and seems to cause the PNG to get ugly.