Please indicate the exact font that you are using, because while you indicated that you are using a Noto Sans CJK font with all 65,535 glyphs, Noto Sans CJK TC, I suspect that you are using a region-specific subset font for Source Han Sans, probably Source Han Sans TW. The issue is with U+9AA4 骤, which is specific to Simplified Chinese, and whose glyph is present only in the region-specific subset fonts for Simplified Chinese, meaning those with the name Source Han Sans CN. I am sure that U+9A5F 驟 is present in the font that you are using, because it is the Traditional Chinese form that comes from Big Five Level 1 and CNS 11643 Plane 1. The easiest solution is to make sure that you're using a font that includes all 65,535 glyphs, such as Source Han Sans TC, and I can virtually guarantee that 步骤—and 步驟—will appear correctly.
If the fonts are being served via Typekit, which I strongly suspect is the case, the reason why Source Han Serif works is because its fonts include all 65,535 glyphs. The Source Han Sans fonts that are being served are the region-specific subset fonts, which have significantly fewer glyphs. When Source Han Sans is updated to Version 2.000 later this year, the fonts served via Typekit will include all 65,535 glyphs. For now, you will either need to use the Simplified Chinese fonts, or download and install the fonts with the full (65,535) glyph set from the open source project on GitHub.
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