555 timer internal SR flipflop

There is no such rule for an SR flip flop. Just don't expect the two outputs of an SR flip flop to be opposites of each other if you assert S and R at the same time. Depending on the construction of the flip flop, either both outputs will be high or both will be low. And stopping the dual assertion at the same time will produce an indeterminate result with the two outputs returning to their normal, opposite states in one of two possible pairs.

Since you are dealing with only one of the two flip flop outputs in the 555, you have to get into the structure of the internal flip flop to determine what happens when you assert S and R at the same time.

This web page and tutorial shows an approximate schematic of what is inside the original, bipolar transistor version of the 555:

The flip flop is made up of transistors Q16 and 17, two inverters connected in a loop. Q17 drives both the push pull output stage (Q20-24) and the discharge switch (Q14) so it is the output you can see from the outside.

The reset input drives Q17, directly, overriding whatever Q16 is doing, so it also overrides and set and reset inputs from the two voltage input comparators.

The trigger comparator's output is Q15, a switch to ground that turns off Q16, regardless of whether it is being turned on by the positive feedback from Q17, or by the current sourced from the threshold's comparator output, Q6. So trigger overrides threshold.

Unless I have gotten tangled up.

Not at all- the S/R inputs are deactivated when Reset is asserted and a very common configuration uses TRIG/THRESH tied together making the 555 into a very good ratiometric comparator.