The reason you are running into this is simply because the underlying fonts most likely have very difference font metrics where for instance, the Line Height, Ascent and Descent lines can be very different. This would most certainly affect line spacing and even character positioning.
Fortunately, you can adjust these metrics in the font asset inspector to normalize these to make sure they work well together.
In the image below, I added the TMP_TextInfoDebug.cs script to the text object to show these metrics. This script is included in the TMP Examples & Extras.
In the above image, the first “AB” is from LiberationSans and the second “AB” from NotoSansCJK.
As you can see, these two fonts have very different metrics, especially when it comes to their Ascent Line and Descent Line. This would affect line spacing as well as vertical positioning of characters on individual lines.
Line spacing is based on lowest Descender of previous line + line gap + highest Ascender of next line as you can see below.
In the image above, you can see how top alignment is based on the highest Ascender. Line spacing is from the lowest Descender + Line Gap (hard to see since it is small) + highest Ascender.
In this next image, I adjusted the Ascent and Descent Line metrics of NotoSansCJK to match those of LiberationSans. Typically, you want to adjust the metrics of the fallbacks to match the primary as seen in the image below.
In this above image, the Ascent and Descent lines of NotoSansCJK have been adjusted which results in consistent vertical alignment and line spacing regardless of the text using LiberationSans and / or NotoSansCJK.
Note: Font come in all shapes and size where these metrics are defined by whoever designed the font.
