Concatenation feature in DocumentArray is broken

[Jackmin801]

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Example code is taken from here: https://docarray.jina.ai/fundamentals/documentarray/construct/#construct-from-multiple-documentarray

from docarray import DocumentArray

da1 = DocumentArray.empty(3)
da2 = DocumentArray.empty(4)
da3 = DocumentArray.empty(5)
print(da1 + da2 + da3)

da1 += da2
print(da1)

Expected Behavior:

<DocumentArray (length=12) at 5024988176>
<DocumentArray (length=7) at 4525853328>

Current Behavior:

<DocumentArray (length=12) at 140264677317840>
<DocumentArray (length=16) at 140264677380944>

Reproduction of the bug in colab environment (with a bit more experimentation on what might be going on): https://colab.research.google.com/drive/1XySeJY_A7d5rf7n0FIa27snBxJawrASR?usp=sharing