I need to be able to utilize #'s with tokens in subject lines. This process is how we populate lead ID's and reference throughout our communication strings and CRM.
On checking with my internal team it seems that this is a known behavior when using # or $ characters along with tokens. The workaround is to try using a different character and see whether the email is getting approved.
Except that it (a) isn't reproducible for me and (b) is always work-aroundable without having to use a different character.
Heck, you could have a Velocity {{my.token}} that just contains
${esc.h}${lead.fLeadID}
and use that.
Open a brand-new email (you can use the Personal Note template as I did above) and try the Subject line. If that works, as it does for me, then the problem isn't the Subject.