Fixed by fix: improve reliability of dead key and compose key handling with the overlays (!126) · Merge requests · Plasma / Plasma Keyboard · GitLab . Noted as fixed in 6.7.0 but actually didn’t make it through before release (noted as 6.7.1 in the last “This Week in Plasma” blog post).
Before then, the workaround is disabling the feature of holding keys for accents or Plasma Keyboard altogether. See: