Certain custom fonts are shifted 'up' in the PDF due to wrong ascender metric

Certain fonts are shifted 'up' in the PDFs. I've noticed this in the Google Noto Symbol and Naskh Arabic fonts, but not in the standard Google Noto Sans font.

If I generate the metric XML file for the three fonts, and copy the value of the <ascender> and <descender> element from the Noto Sans font to the metric files generated for the Symbol and Naskh Arabic fonts, those two fonts are then 'fixed' and appear in the correct place again.

(Not that the Naskh Arabic font is overflowing its line at the bottom, so there is still something wrong there, but a lot better.)

To reproduce:

1. Download the NotoSans-Regular.ttf, NotoNaskhArabic-Regular.ttf and NotoSansSymbols-Regular.ttf font files and put in a folder.
2. Download the attached input.fo, FOPconfig.xml, NotoSans-Regular.xml, NotoNaskhArabic-Regular.xml and NotoSansSymbols-Regular.xml and put them in the same folder.
3. Run fop -c FOPconfig.xml input.fo output.pdf
4. The output should look like the attached output.pdf file, showing the issue.

The three metric XML files were generated using the instructions on the Apache FOP Confgiuration page, and I then edited the NotoNaskhArabic-Regular.xml and NotoSansSymbols-Regular.xml by overwriting the <ascender> and <descender> elements with those found in the NotoSans-Regular.xml. Note that I don't know anything about the metric files, so I don't know if editing those elements were the correct thing to do, or even what they mean!