When holding glass in first person the block is transparent to all things in the world. The same is true in third person except for some edge cases listed below:
water (source and flowing)
clouds.
slime blocks
glass panes
glass blocks
honey blocks
When viewing the object through the glass block you're holding in third person these blocks/things are invisible.
It should be noted this only affects third person view, first person appears to work for all listed cases.
Thank you for your report!
We're actually already tracking this issue at MCPE-55631.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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