Vowel in the "Back of the Mouth"

[XiaoH]

I noticed when I record songs I have this constant problem where sometimes when I sing vowels, it sounds like it is "placed" in the back of my mouth/throat. Although when doing exercises, I am supposed to use a very open vowel and sort of place it in the back with a lowered larynx to help train tone? Or is this wrong. Also, when training higher notes (siren up) I do the same thing/ drop jaw really low, but i guess doing this for higher notes is correct to help train it?

Examples: a sustained "Night" (N - aaahhhhh - eee -ght) singing the ahh like yawning

- "Far" ( Fa - ahhhhhh - arrrrr) again, singing "ahh" and "arr" sort of like yawning

Can someone provide me an explanation for this. I noticed that my tongue is placed more closer to my bottom teeth when this happens, and my jaw is dropped. Should my tongue be placed closer to the top teeth when singing these vowels and doing exercises with open vowels? Thanks