Reindexing is the process of using blocks that already reside on disk to rebuild the "chainstate," i.e. the leveldb databases that represent the chain of blockheaders and state of the UTXO set. It is essentially initial block download, but without having to download blocks from the network.
A user claims that "performing a reindex of the block files reliably introduces spurious bytes, resulting in subsequent deserialization errors," as detailed in the original issue. The reporter has included many details pertinent to the reported failure.
Investigate the failure, determine if reproduction is possible, and devise a fix.