@Christof It sounds like you're not aware, but as NancyFox writes several times in discord there seems to be some internal discussion/work on this already:
"We are working on a solution to this problem. We hope to fix this soon." https://discord.com/channels/416536675989323776/486932129800781825/825624117150482443
"Many people ask to make sure that the entrance to the arena cannot be refused. My personal opinion is that the introduction of this function will lead to the fact that players will be more serious about submitting applications to the arena." https://discord.com/channels/416536675989323776/486932129800781825/825651774341775380
"This problem cannot be fixed in 2 days, sorry. Our programmers try to do their best. However, on the PTS client, the changes made are minimal and they must be made very carefully so as not to damage the system and not make even more mistakes.
We hope for your patience and understanding." https://discord.com/channels/416536675989323776/486932129800781825/833599750128795668
As Argothon linked above, the topic has been brought up several times with several different suggestions for how to fix it. Being instantly ported inside is probably the best solution (if you have to go afk you have to cancel queue) since people will be forced to fight if they want their entry's worth, but other servers have also implemented getting the arena ticket refunded if no opponents. This wouldn't fix the issue in 6v6 and 12v12, though.