There may also have been some changes (the usual ‘not in the patch notes’ type ). I remember a while back being able to hit loose branches with a pick and collect several. Then it seemed like it had changed to only certain branches (there was a point where I could recognise them by shape, but it’s been a while). Then there was a patch and it seemed that if I hit even the ‘good’ branches with a pick, they just vanished - but trying it with a hatchet started working instead. It’s possible that’s still the case, or they may have changed ‘the rules’ again.
For loose stones it seemed to remain more stable - I never was able to get anything out of the smaller ones (whether they didn’t work by the time I started, or I just wasn’t able to line up on the hit box I have no idea). But the mid-size stones (the larger ones that you can pick up) still seemed to be working last time I tried. Location didn’t seem to matter, but like you say the sheer numbers in the volcano make that a good spot.
I never tried to hit lotus with a pick or hatchet - hitting basic bushes with a pick always seemed to work well for getting insects without getting any plant fibre, so I guess I just assumed it wouldn’t work on lotus. Be interesting to give it a go to see what happens.
To the best of my knowledge it doesn’t grow anywhere else in the EL.