Hi,
I have several sites where I use JEM, some are test sites others are production sites.
On several of these sites the nl-NL package was still at version V2.2.0, so I started to upgrade them towards V2.3.3
ALL of them failed to install 2.3.3 with the missing manifest file message.
Luckily I still had de V2.2.3 nl-NL package in my personal repository so I tried that one, and all these sites installed V.2.2.3 correctly.
All these sites run Joomla V3.10.2 with php 7.4 or 7.5
Maybe you can mail or wetransfer me a known good nl-NL zip-file to try out.
Regards, Johan.