Formal-sounding substitute for "stuck at"

Here is the sentence I’m working with:

Product manufacturing has (been) ________ (at) one unit per month for
nearly a year.

Clearly, if just any old word/phrase would do, we could insert "stuck at":

Product manufacturing has been stuck at one unit per month for
nearly a year.

I also considered "limited to," but it oddly seems too out of place as well: there is no regulation or anything from keeping product manufacturing from going above one, it’s purely a discretionary decision.

Is there a more formal-sounding substitute for "stuck-at" I can use here?