Hi @Mathiau,
That's an interesting question! I don't know if TP-Link strictly enforces 802.11 protocol limits. It could also be a bug/limitation in inSSIDER -- you might email.
Do you happen to have any old b or g clients to see if they are actively rejected when the EAP is set to "N" only?
I have my 225V2's set to n/ac on the 5.8 because I don't have any "a" clients. But I keep my 2.4 on b/g/n b/c some of my IoT stuff is only b/g anyway.
I havn't found that leaving extra protocols enabled really has any impact on performance. I would, however, limit your 2.4 to 20MHz only. And I would force 80MHz for the 5.8. I have seen some strange behavior, occassionally, with the 20/40/80 setting.
-Jonathan