Hmm.

You have a reasonable amount of dead weight in the text, for example the entire paragraph

Any individual found exhibiting symptoms of SCP-XXXX will be immediately reclassified as an instance of SCP-XXXX-1. SCP-XXXX-1 are apparently unaffected by use of amnestics; it is therefore required to keep them under a high level of security. SCP-XXXX-1 specimens are to be kept quarantined in a standard Humanoid detention cell built in Site-45, isolated from any other staff or SCP-XXXX-1 specimens. They are to be fed three times daily; a separate common area of 100m x 100m in length and width will be allocated to them for 30 minutes every day. This is provided for therapeutic and athletic use only. This can be revoked if necessary, and can be used to coerce SCP-XXXX-1 specimens into compliance.

could be replaced with

Individuals exhibiting symptoms of SCP-XXXX are to be quarantined.

This is because the details of quarantining are unimportant and can mostly be guessed; they don't reveal any useful information. There are other redundant parts too, for example you explained that those affected by SCP-XXXX are reclassified as SCP-XXXX-1 twice.

Aside from phrasing and delivery:

I don't think you need any kind of additional data to explain the "subtext" (I think you mean "implication", "subtext" is something which isn't explicitly part of the story). That was pretty clear to me. In particular, the line "many Foundation staff believe SCP-XXXX is a work of fiction" is a complete giveaway.

I was going back and forth over whether you should keep the framing device of the lead-in and conclusion, but I'm leaning towards stripping those parts out and leaving this as a plain old SCP. I think you're kind of giving the game away with the extremely mundane bullet pointed list of symptoms of SCP-XXXX - it is a little too obvious that all of those behaviours are completely normal human behaviour for a dissatisfied worker in a workplace like the Foundation. You should remove about half of them, especially the particularly mundane ones like "A general lack of any useful activity".

I think it might be more effective if this leaned over into anomalous territory. There needs to be some weirdness. Maybe put some genuinely anomalous behaviours on the list, maybe minor behaviours which would not be noticed unless you were looking really carefully. Maybe some things which are really easy to frame an employee for doing? Excessive use of Dilbert cartoons in one's cubicle. Stealing lab chemicals.

Maybe add some odd containment procedures which only make sense if the people being contained have anomalous behaviour?

The other way you could go is to flip this around and make it an "official" corporate email. Imagine like those "Loose Lips Sink Ships!" posters or weird corporate motivational group exercises. Instead of trying to pretend that SCP-XXXX is real, make it an official email to new starters. "The most dangerous SCP of all: laziness!" and so on. That could be pretty entertaining. Actually I think you might find more success in this direction. You could still post it in the main listing, incidentally! There are totally valid in-universe reasons why the Foundation might put tedious training blurb right there in the database.