Excel 2021 is making a simple task harder than it needs to be, and that's frustrating when you're trying to build something as personal as a herpetology masterlist. The greyed-out "lock aspect ratio" checkbox in note images is a known quirk of this version, not a sign that you're doing something wrong. You've done the right thing by troubleshooting on your own, searching forums, testing workarounds, even revisiting steps that worked before. The problem isn't your technique; it's that Microsoft changed how notes handle image sizing in this iteration, and they didn't bother to tell anyone.
Here's what's happening practically: in Excel 2021, when you insert an image into a note, the software defaults to a fixed aspect ratio that ignores your manual adjustments. The checkbox being greyed out means you can't override that default through the interface. But there's a workaround that doesn't require add-ins or VBA coding. Before you insert the image into the note, resize it in an image editor, even the basic Paint app works, to match the exact dimensions you want in the note. Then, when you paste it in, it will retain those proportions, and the squishing won't occur because the image is already the right shape. It's an extra step, but it keeps your notes clean and your photos looking like the reptiles they represent.
Your instinct to avoid placing images directly in cells is spot-on. Direct cell images shift with sorting, overlap adjacent data, and turn a tidy observation log into a visual mess. Notes, on the other hand, keep everything tucked away until you hover, exactly what you want for a reference list. So don't abandon that approach. Instead, treat the image editor as a pre-flight checklist: crop, resize, then insert. It's not elegant, but it works, and it's the kind of pragmatic fix that keeps your workflow moving without waiting for Microsoft to patch a feature they clearly deprioritized.
One more thing: you mentioned this worked before. That's not a fluke, older Excel versions allowed aspect ratio locking in notes, and many users have reported the same regression. So you're not misremembering, and you're not alone. For now, the resizing workaround is your best path forward. It's a small price to pay for a masterlist that actually looks good when you hover over a species name. Keep building that list, your future self will appreciate the clarity when you're identifying a frog at a glance.