Affected versions

Irssi 0.8.17-beta up to and including 0.8.19 up to 0.8.19-219-g52fedea

Bug 1 affects only Irssis compiled with true-color enabled.
Bug 2 affects all Irssis regardless of compilation flags.

Fixed in

Irssi 0.8.20

Recommended action

Upgrade to Irssi 0.8.20. Irssi 0.8.20 is a maintenance release
without any new features.