This is with:

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.23)
 of 2020-12-14 built on xo
Repository revision: 86d8d76aa36037184db0b2897c434cdaab1a9ae8
Repository branch: HEAD
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Debian GNU/Linux bullseye/sid

Which is quite similar to your build:

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.22, cairo version 1.17.3)
 of 2020-08-28 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Manjaro Linux

Let's see...  the main difference seems to be that my 27.1 build is
without cairo?

Nope, even when building with cairo, I'm not able to reproduce this.  So
my guess would be that a font library in your distribution has a bug in
this area?

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