Polymer seems to assume that certain tags in block entities always have a certain format here, without considering that mods might use different formats.

For example, the shadow drawers from extended drawers contain an item tag, but it doesn't follow the same format as vanilla items, instead it's a serialized ItemVariant. This causes the above mentioned code to error resulting in log spam and potential loss of a few packets (which don't seem to matter for extended drawers).

Fixed in 0.11.3+1.21.4