It would be interestinh to know why it is this way. I will re-srrange the inputs
Hi
@LeoB - have taken interest in this, mainly because I myself have lengthy experience w/2488's and my O/G DP-32 (which this forum category is dedicated to) - so I thought I could help. Normally I'm the guy who has to remember which way the pointy end of the guitar goes every time I sit down to play.

If you're smart, you'll listen to guys like
@Mark Richards and
@-mjk- and
@Phil Tipping.
FIRST OFF:
no, it doesn't matter which input your mic is attached to, as long as your specific model MTR has the right connector at that input (for example, the 2488's have some XLR's, and some 1/4" inputs) AND chosen input has +48V if your mic requires it. My DP has XLR's AND +48V at
all inputs. You can, of course, assign input A to any mono track!
ANYway:
Mysterious as it is, the limitations on "pairings" of inputs you've experienced are clearly by design, as they're outlined in the O/M's. At least they addressed it - in recent years, Tascam O/M's are about as functional/worthy of existence as the US Congress. Considering how unhelpful they are (especially to beginners/novices) I imagine they're translated to English by the alcoholic janitor in between drunken binges, late on a Friday.
On the basis of other limitations I've experienced or seen here on the forums, I'd dare suggest these are limitations created by the processing power capacity of the units, or perhaps the limitations on the specific routing capabilities imposed by the internal electronics design capacity/limitations. It likely saved them cost and/or design requirements by not making the routing infinitely variable, but rather providing the unit/s with the capacity to use typical, utilitarian routings.
As if anyone would care - my routings on my DP-32's 8 ins are:
A & B: from L & R main outs of my mixer (all my instrument/mic sources run through)
C & D: from SUB1 & SUB2 of my mixer (for separate output while recording multiple sources)
E & F: I leave open, for times when I want to plug in mono or stereo sources directly to the DP (which I routinely do, for a variety of reasons)
G: normally open, "spare"
H: left open as much as possible, to be free for direct connection of a git'r for use of the DP's internal modeling/FX