Commands in chains do not run sequentially

Builds tested on: 586644, 547645, 408727

Summary:
Within a chain of chain command blocks the commands all run in the correct order, but if there is another chain nearby their execution appears to be 'interweaved', as in you may have a command from the first chain run, then one from the second run, then two from the first run, etc. What is even worse is that is some cases the order of this 'interweaving' appears to be semi-random. Which command from the two chains that runs first differs between ticks.

Impact:
This greatly reduces the effectiveness of having multiple chains, and its unreliable nature makes creating even simple multi-chain systems (which are sometimes necessary to enable turning different sections on and off) difficult, if not impossible.

Repro Steps:
1. Create two parallel adjacent command block chains such that:
Chain 1: say 1, say 2, say 3
Chain 2: say 4, say 5, say 6
2. With a lever and redstone power both chains at the same time.
3. Observe chat.

Observed Results:
Something along the lines of: 412536

Expected Results:
Either 123456 or 456123.

Screenshots/videos attached:
Yes, MCM-157.mp4. I could not reproduce the semi-random behavior but the core issue is demonstrated.