First char not highlighted

[agenbite]

I'm observing a weird behaviour in which the first character is not properly highlighted in some, apparently random, cases. Here's what I see if I call avy-goto-char-timer on the customization page for avy. As you can see, some of the first characters are not highlighted, while others are. This is apparently not related to the type of text, since it happens on the descriptions for some options and not for others.

Is this a bug? How can I help (as a n00b) in resolving it?

EDIT: This happens on the current Emacs master. Not happening in Emacs 28.0.90.