Yeah, I think that explains it for the wheat. The phenol is 4-vinyl guaiacol, and is desirable in a hefe. It's the same compound that gives cloves their flavor. Higher temperatures favor its formation.
Also, stressed yeast can lead to other phenolic compounds, which may be what you're tasting in the other beer. It's ok to not do a starter with a lot of beers, but it increases ester formation. However, if you forego a starter for a moderate gravity beer, it's absolutely necessary to oxygenate the wort beyond pouring and shaking.
If you want to avoid this in the future, either look into starters, or get an oxygenation system. They're not that complicated - NB sells a nice aeration wand and regulator set-up, and the small O2 canisters from home improvement stores are perfect.
To make a starter, you really only need a large jug, like a gallon apple juice jug, and some DME. Stir plates and flasks are absolutely not necessary. Ask @loopie_beer, he's a convert to the shaken, not stirred method!