I feel you, I am subscribed to hundreds of open source projects so it may sometimes take more that two weeks to follow up. Discourse already has “this topic is old” warning to discourage necro-bumps so I feel force closing is more annoying than necessary.
Regardng the error, Meson 0.54 started caching dependencies so it would notice inconsistencies like this. Perhaps GTK/Pango still did not catch up? Try downgrading Meson to 0.53.2.