Beginning today (May 3, 2025), all models (4o, o3, o4-mini-high, and o4-mini) are, without exception, escaping single $ signs (especially bad for the use case of inline math notation). In the past, some of these models would try to use \[ escaping for inline math, but could be instructed not to do so.
This new “feature” (bug) seems impossible to work around. Even when explicitly asked to mimic e.g., ‘$x$’ or to explicitly add a specific number of backslashes, the thought process still prefaces each $ with a co-joined backslash. Even in non-latex output, all single $ signs are escaped (e.g., output like: “The apple costs \$1.10”).
Note that for the purposes of the post, I have intentionally added backslashes myself so the post is readable. 
This applies whether logged in or logged out.
This bug seems brand new today; I’ve never seen it before, but it’s a nightmare.
As a note, copying gpt responses from yesterday to the beginning of my chat history does not show any verifiable instances of this happening before.
In addition, in canvas mode there is also no way to remove escaped characters and replace them with non-escaped versions. Consistent math output has been very hit and miss in general. Deep research reports always escaped everything making them unreadable, but at least regular chats used to render mostly fine.