From your screenshot it looks like your using a plugin that exends TheWire (I think TheWire Tools). That's why I asked:
are there any plugins that effect TheWire?
I tried your steps with only TheWire from Elgg core active (no additional plugins) and no issue with replying to a Wire post.
I then tried it with TheWire Tools enabled and got the same issue as you described. So the issue is with TheWire Tools and not with Elgg.
I'll look into the issue with TheWire Tools.
I fixed the issue in TheWire Tools, see https://github.com/ColdTrick/thewire_tools/commit/84a332f7e5b16cf007e765bbdc0a542e73424187
Please note: this will only apply to the Elgg 6 version of TheWire Tools since we don't support our plugins on previous versions of Elgg.
I will wait to update when all the plugins are updated and available to the Elgg 6 version.
Thank You
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