Exact Problem:
The NBT tag SuccessCount on commandblocks has a delayed update to SuccessCount:1 and vise versa. By the looks of (MC-87611), only one block gets updated at a time in a commandblock chain.

Note:
This causes conditional commandblocks to function incorrectly.

How to Replicate: (View attachment for detail)

/testforblock ~ ~-1 ~ minecraft:chain_command_block -1 {SuccessCount:0}

When running the following, it should only return true / successcount:1 when the block below /testfor @p[r=5] is false / successcount:0

Problem:
With /testfor @..., it first returns SuccessCount:0 before updating to successcount:1, making /testforblocks... {SuccessCount:0}'s condition true at least once.