As soon as any of the fixed Sub outputs is selected - regardless of the Main selected or not - the very same signal is sent to Mix 9 and Mix 10 (it has the same behavior in all 3 FlexMix modes). I tried with the Tape in coming from the computer on USB, also on a random input with a mic.
Are we dealing with a bug here?

No, can't reproduce that on my SL24III. Believe when something will send to a FlexMix
Bus without setup it it would be already see by others.
Can you describe like Step by Step what you setup up ?
Are you sure that you Reset Digital Patching / Project successful
and not use any old not compatible settings pre Firmware 2.x ?

Also, if I may have a simple question here... is there any way to have these built in Sub outs Post fader? Meaning that I would like to be able to ride their volume with the Main out Fader. I hope it's clear for everyone.

They are anytime Post Fader, but not Post MainBus Fader only Post the channel fader.
There is no "after main Fader" and maybe there is no mixer in the Universe with that.
You have your SubGroup A-D for that.

And anything else make not really sense or only in very special situation.

Please describe what you try do do...

What I was trying to figure out is a good way to feed a separate sub speaker and say a pair of delay speakers, and be able to control all their volumes (including the mail LR) with only the Main fader. There is a way doing that with Matrix mixes, works like s charm, but you’re burning flexmix channels. So I thought how cool would it be to do it using the 4 built-in sub buses.

You should tell us what mixer you are using, because solutions are different for consoles and rack mixers. If you want to send a sub signal from FOH (console) to the rack mixer on stage (in stagebox or monitor mode), you will always have to sacrifice one flex mix, because you need the phsycial output. A matrix mix is the way to go.

Still you should think of other options. A subgroup or an aux sub are very good alternatives.

15 aux mixes are not enough...?

The nature of subgroups is to be postfade, because they only serve as volume control and processing step for "grouped" input channels. Therefore they are - by default - routed to main mix. It is, of course, possible to unroute subgroups in order to use their physical output (available on models SL 32S, SL 32SX and SL 32 only).

In flex mix subgroups, you will find all input channels available. In fixed subgroups, you have to select the input channels you want to be part of the subgroup. In UC Surface you click on the input channels and confirm the "editing" by "Done". On the mixer you use the touch screen (right down corner) and the letters A, B, C or D, which represent the fixed subgroups. If you want to group a subgroup (A+B and/or C+D), this needs to be done on the mixer. UC Surface 3.0 does momentarily not support this feature...

I don't see any way how any of the subgroups can affect flex/aux mixes 9 and 10. There is no bug. I have no idea what could cause aux 9 and 10 to "mirror" a subgroup. Maybe you changed a subgroup to an aux, or vice versa. Fader settings are always steady.

The new "flagship" of the Series III mixers, the SL 64S, has got a dedicated mono output, which can be used to feed a mono sub, but the rack mixers or stageboxes you need to add to the system, do not offer enough outputs (should be 32+2+1). Again, you need to sacrifice a flex mix...