Anyone else experiencing some of this?
I recently recovered (barely) from a near disastrous instance when apparently the symbols lost their linkages to the images w/in the animation.
After working for several weeks on an animation w/ symbols w/in symbols I was nearing completion when I noted several anomalies in the animation while testing.
Missing images, triggers that didn't seem to fire, truncated animation in the browser (chrome) and weird positioning and hiding of images and animation in AE's UI.
Admittedly some of this may have been my fault for tweaking images and image naming outside AE, but it seems there were other things that corrupted the intial AE *.htm animation file and its associated *.js files/linkages as well.
Regardless a few things I noted that might help someone else trying to recover a project gone awry.
* Symbols *
Export a copy of your symbols to a folder that you can use as a backup just in case your symbol *.asym file gets messed up.
Update that file when you're sure your animation symbol is working properly after major edits.
Use symbols & export them when you have a working segment for your animation. It can save grief later if you have to rebuild a complex project.
* Images *
Avoid fiddling with images outside AE.
Changes to filenames are usually okay as long as you use this sequence: Outside AE (windows explorer) copy your original file and rename the copy. In AE open *.htm and the relevant symbol.
Go to the double horizontal arrow in the properties of the image and click it. Select the renamed copy image (your new image file). Save the *.htm in AE. Close AE. In windows explorer delete the old image file. When you open AE again it will complain that the image in your library is no longer there, but you've already pointed the image instance to the new file. So no worries.
If you have an image that has disappeared from a symbol.
Open the symbol to edit it.
Deselect everything (no layers selected).
Drag the image you used to have from the library onto the stage and position it as desired.
I noted in several cases that doing this actually "awakened" the symbol to recovering timeline changes and tweaks that I thought were lost -- that is, the timeline for a symbol with a missing image was entirely blank for that image. I dragged the image back onto the stage while editing the symbol and the timeline work for that image reappeared. (Yes, very weird.) So I went from seeing only an empty square box and an empty timeline for the symbol to one with the image as well as the timeline animation.
* Truncated Animations in Browser *
Here the animations didn't complete, didn't start, or sections didn't run. I could add "other weird behavior" to the heading including the AE UI showing a symbol and its animation as a small box positioned at the top of the stage or hiding animation when viewing the full timeline vs playing from the edit symbol position.
Here I simply said it was too weird to expect AE to recover the file. BUT because I heavily used symbols for sections of my animation, all was not lost. I started a new AE animation file (*.htm), setup the stage to match my original project, then dragged the symbols for each segment onto the stage one-by-one checking first if the symbol still was tied into its images and still otherwise worked. After confirming the symbol was setup properly I saved the AE file and continued to the next symbol until all were back on the timeline and running as they should be both from the UI and browser.
Next I resetup the preloader and downlevel stage.
While fixing the symbols, I reconnected any images that didn't import properly within any symbols (see images above), before adding the next symbol.
In the end, while symbols seem to be somewhat fragile, they can also be a life-saver in times of corruption in the main animation. Plugging in a handful of symbols representing sections of a project is a whole lot easier than doing the whole thing over from scratch. For me it saved weeks of work, though at least a day was lost while recovering the file from the symbols I had saved.
As for anything with these your-mileage-may-vary, but some of it may be worth a shot in a pinch.