I've encountered a weird glitch I had from time to time in the last few AE versions:
(Current Version 18.1.0 on Windows 10 - 19041.928)
I'm bulding quite massive EG Panels for my company as aet templates, so far so good. But sometimes, when I load up a saved aep project again, some elements are missing completely from the panel. They are still there in the comp, but not in EG. I also can't create groups any more (comments work), and sometimes - but not every time - when I drag a single object or group in another group the missing elements reapear one by one again...
And right click/show in EG Panel does not work at all with those elements.
I have occasionally encountered properties disappearing from the EGP. Closing and re-opening the EGP helps - sometimes. At other times, it requires restarting AE to get the props back in place, in the EGP. This issue only happens with highly populated EGPs. I can't be certain but this issue may occur when AE gets confused if you have more than one composition opened and they have identical names. HTH
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"highly populated EGPs." - you name it 😉 Just an excerpt, more with scrolling...
" if you have more than one composition opened and they have identical names." should not be a problem, thats something I can preclude with 100% certainty as my templates also use extend script panels that would not like duplicates either 😉