This happens when "Update Up next automatically" is enabled and when episodes are being added with "play next". It is not reproducable when the episodes are added with "append to Up next". So there must be an issue when episodes are being added between the episodes of "Up next".
Latest beta 23.8.4b
See screen recording: https://c.gmx.net/@334288198261801446/372bMPzGQaKP7UceX_HTxA
This happens when "Update Up next automatically" is enabled and when episodes are being added with "play next". It is not reproducable when the episodes are added with "append to Up next". So there must be an issue when episodes are being added between the episodes of "Up next".