Sorry to bring this back up but this feature does seem to be working strangely. I noticed this my first time playing a campaign well into the late game, and have noticed it in every single match since. The problem, unless you look for it does not rear it's head until the late game where it is impossible to maintain a positive credit income with a full fleet (maxed out empire tree). So I sat down and decided to try and figure out what was happening and why.
I started a new game on the Dissolution map with four normal AI players and myself. My intention was to see how the effect was late game but I noticed it fairly early. I was playing as Imperial Remnant and had yet to do any empire research so had the base 3 command and 60 supply with upkeep at 0%. My fleet consisted of 3 ISD Mk Is, 1 Dreadnaught, 2 Escort Carriers, 2 Lancer Frigates, 8 Carrack Cruisers, plus the two Mu scouts leaving me a total of 14 unused fleet supply. According to the info cards, the ISDs each cost .2 credtis/sec, and the rest .0 credits/sec. Since I cannot actually tell how much that adds up to I assign .05 to the .0, which gives a cost of 1.15 credits/sec for my fleet, which if triggered every 5 seconds as mentioned above would come out to 5.75 credits/sec.
My credit income was entirely from planetary taxes and gross tallied at 18.2 credits/sec from two planets. However, my net income would start at the gross 18.2 credits/sec and would drop by .1 credits/sec every half-second until it reached a sum of 8.6 credits/sec and then it would reset to the 18.2 credits/sec and begin the cycle again. This, however, does not seem to actually hold over into the actual tics and amount of credits received. So, in terms of actual credits received I followed the tics and tracked them: starting at 864=>859=>873=>868=>882=>877=>890=>886=>899=>894=>908=>903=>917. So over six tics, being six seconds, I tracked each interaction. Each tic I would gain 14 credits and then lose 5 credits before the next tic, however, my credit income should have been 18.2 credits/sec with a loss of maybe 2 credits/sec from what I could see in game. If the upkeep payout occurs every five seconds as accumulated over that period I ought to lose up to 10 credits every 5 seconds, however, this does not appear to be the case. Over the course of five seconds I had lost 24 to 25 credits on top of the amount already deducted from the gross income of 18.2 credits/sec which was about 4 credits/sec lost additional. In total, over five seconds I lost 44 to 45 credits total as opposed to the roughly 6 credits that, according to what I can see on the infocards, I ought to lose every 5 seconds. This issue only grows as the game continues, until in the late game it seems to be impossible to have a positive income.
I hope this makes sense, and that I explained the problem as clearly as I could. Furthermore, I am running Ascendancy 1.15X with menu patch with income rate maxed, research speed maxed, build speed maxed, and all other speeds at default.