I have five fonts, in which some glyphs are identical across all fonts but have different vertical metrics in one of them. I have generated them as one .ttc file with the most recent version of FontForge (2023-01-01).

When I open, from the .ttc file, the font in which they are different, the vertical metrics have been altered to that of the other fonts.

For example, take （ U+FE35 and ）U+FE36. Originally, across the fonts,
（　has different vector data and different metrics,
） has the same vector data and different metrics.

In the generated ttc,
（ has correct metrics.
） has mistaken metrics.

I used the ttx command from fonttools to inspect the .ttc file, and the vertical metrics table (vmtx) shows:

<mtx name="uniFE35" height="4096" tsb="2899"/>
<mtx name="uniFE35#1" height="2048" tsb="850"/>
<mtx name="uniFE36" height="4096" tsb="119"/>
<mtx name="uniFE37" height="4096" tsb="2737"/>
<mtx name="uniFE37#1" height="2048" tsb="688"/>

The character map table (cmap) shows:

<map code="0xfe35" name="uniFE35#1"/><!-- PRESENTATION FORM FOR VERTICAL LEFT PARENTHESIS -->
<map code="0xfe36" name="uniFE36"/><!-- PRESENTATION FORM FOR VERTICAL RIGHT PARENTHESIS -->
<map code="0xfe37" name="uniFE37#1"/><!-- PRESENTATION FORM FOR VERTICAL LEFT CURLY BRACKET -->

There should be a uniFE36#1 in (vmtx) and (cmap) should link to it...

I think the TTC generation should not only check glyph similarity but also vertical metrics!