Thanks for this post, wild_flower32. I recently discovered that I was also closing my jaw significantly, and I think it is related to my pronunciation. This is an international website, and others may have this same issue based on your language and accent.

When I sang “mmm… ahh… ooh,” I was pronouncing the “ooh” with a sound like the following phonetic:
[uː] you, who, chew, shoe, cool, tool
For my American accent to make these sounds, my jaw would rise unconsciously.

After finding this point of difference, I now sing “mmm… ahh… OOOO” with a long “O” sound. Again with phoentic:
[o:] or [oʊ] go, note, goat, rope

By making this minor change, I noticed my greatest improvement thus far, and I think it is purely an opening of the throat and jaw.