but whenever I ask Alexa to turn on/off the TV I am getting a “unresponsive”-message. Also in the Alexa App it is shown as unresponsive, all other items are working fine. Is this an issue on my end or is this a bug in the openHAB Alexa Skill?
As mentioned in the link below, since you set your TV power item as autoupdate=false, the skill will not report back the state of the given item to Alexa. This will affect the usability of some of the advanced features in the Alexa app that require state reporting.
Setting autoupdate=false usually indicates the item doesn’t have a state or it has a delayed state update. In both cases, the skill wouldn’t be able to get the proper state and therefore it is considered by default as non retrievable. Setting metadata parameter retrievable=true is overriding that logic as you highlighted.