I would note that 1L packets are on the small side for practical replenishment -- though I suppose you could mix one for working solution and a second for replenisher (my working solution is 2L, however, because my sheet film tank takes about 1.6L for 4x5 film). Still, not a bad idea to sell both 1L and 5L size, so folks who use it slower don't have to be as concerned about the stock solution going off.
Yes, it is quite amazing that you can replenish with only a 2L volume. After all replenishment was designed for larger tanks in commercial labs. For my neutral fixer I've just began a replenishment scheme with only a 1L volume, but fixer is a process that goes to completion. I already replenished half the volume in a weekend by finally developing all the b/w film I shot as far back as August.
Developers, OTOH, are precision tools and the engineers will have assumed a minimum tanks size when tuning the replenishment volume to get to the right balance of developer and byproducts that will eventually establish itself. Too small a tank and you should get a different ratio with the given replenishment rate. Luckily overreplenished and, and for b/w that is okay. You'll simply adjust the times until the negatives come out as desired.
For C41 and its fixed process parameters it's not that easy. I hope my 5L are good enough. In a Fuji document there is a chart regarding tank volume, number of films per week and which of their replenishment systems should be used. Their minimum tank size is 10L. Only twice my tank size and hence my hope that the balance will ok. Oxidation may be a reason for the smallest tank size in a minilab and should be less of a problem for me because a minilab tank, even with floating lid, will be much worse at storing solutions than my 5L PET bottles and Butane cover. Same for my Xtol.
Back to the topic:
I've been using a 5L bottle of Xtol and developed close to 20 rolls in it over the last few months. Remember that you can develop up to 5 rolls in 1L without time adjustment. The limit for a 5L batch is 25, then. Using the times for stock Xtol all films looked good.
I began replenishing now from aother 5L batch. I threw away the outer packaging of the 2nd batch just days before I heard about this new problem. For that reason I have no idea if my Xtol has of the problematic dates. I bought both bags in February from Maco direct. Probably because Foto Impex didn't have it in stock, or something else I needed at the time. Maybe I should run a test roll in 250mL of the replenisher before going on further.