With further analysis comes additional confusion:
Toasts (still can't believe that's what they're called...) - I guess that I must not have been playing on my Nexus while this happened previously. When I take a new image or discover a new species, I get a popup that behaves exactly like the PC version. I have not yet finished a task on the Nexus while paying particular attention to this, but when I do next, I will try to do so and confirm that all of the popups are appearing as intended. Apologies for the confusion; I can't believe I had just never seen one after playing on that thing for as long as I had.
Headings on panoramic images. This got weird and helped explain some of what is making me feel like an idiot, since the headings appeared and reappeared after minor changes and in multiple ways.
In all cases I tried to get the headings to appear first by simply looking to see if they were present, then by tapping the image (this made them appear sometimes) then by moving the image, then by scrolling up and down to see if that had any effect.
Additionally, it is not just the headings that are disappearing but the entire overlay. This includes headings, full screen button, existing image tags, and scroll buttons at the sides of the image. I can still scroll the image by dragging it left and right.
a. Brand New Image 1 had no headings when I clicked on it with the toast active. Clearing my browser cache and refreshing did not activate the headings on that image. Eventually the image gained headings (I honestly have no idea what happened to make it so.) When I added a tag (without submitting ID) the heading disappeared. When I submitted it, they stayed gone for at least half an hour (after the species had been IDed.)
b. Some other images still had functional headings, but others did not. I've kept one panoramic image without any tags at all, and it has gained and lost its heading several times.
c. I found an image that had headings and free space for tags. Once I added a tag, the headings went away. On the PC version, they flash away, the box is drawn, then they reappear. It was apparently the reappearing part that was not happening. If I cleared the cache and refreshed, the headings came back some of the time.
d. After a suitably random period of time many of the images that have lost their tags have had them reappear.
e. Brand New Image 2 had headings when I checked it without the popup present. They lasted for about 10 seconds and then disappeared without me touching the image or adding anything, just scrolling up and down a bit while they were always on screen.
e. New issue - When I move or tap an image that has tags, the tags disappear. They reappear as long as I am holding my finger on the image.
f. New issue - If I hold the side scrolling button on a panoramic image, the button can disappear and the image gets stuck spinning in circles. It can't be stopped other than by refreshing or Back/Forward.
I've now run out of images with functional headings to break. For whatever reason the breaking is persistent. Clearing the browser cache and reloading the page has no effect at all.
These now have the same set of problems that the panoramic images have. None of the headings or buttons appear, or if they do, it is only briefly and then they disappear more or less permanently.
On the plus side, I can now move/resize the new tags on these images.
Going to have a whack at messing around on an iPad for a while now.