Table 20 in clause 7.6.2 of ISO 32000-2:2020 (and 2017 for that matter) states that the Length entry is deprecated, and only meaningful if the security handler version is 2 or 3. Nonetheless, Acrobat and Reader refuse to decrypt files that were encrypted with a v5 security handler that doesn't set this entry. I've attached two files that were generated in the exact same way, barring the absence of the Length entry in one of them.