I worked at one pharma company that maintained policy of the value of keeping batches consistent. I would prep 4 L of a 0.1 M triethylamine acetate eluent, and store half in the fridge. I gave it a (somewhat arbitrary) usage date of one month. Even so, as time passed, this one batch changed it's baseline over the space of one month, even when stored refrigerated.
I worked for another pharma company that always prepared fresh eluent, according to method, for each run. No eluent older than 24 hrs (even 60:40 methanol:water) was allowed.
My point is that you can justify either point of view, as a company policy, and still be within FDA regulatory compliance. So perhaps it's worth revisiting your policies for just this eluent system, or just this analysis, or just this sunshiny system.