Thanks so much @Ferry! Your recommendation got me steered in the right direction.
I added the :Ymd to my datetime field and also had to remove the acf_ prefix from the start of the meta key. I had inappropriately copy/pasted “acf_datetime_expire” as my meta key when I only needed “datetime_expire”, in my case.
My final meta queries look like this (for anyone needing to replicate something similar):
This allows my Announcements CPT to be prewritten and automatically become visible on the publish date/time and then hide once the expiration date/time comes.