In 22w46a and earlier, this command just displays Hello:

/tellraw @a {"translate":"%s%4363$s","with":["Hello"]}

In 1.19.3-pre1 and later (including rc1) it displays the raw string %s%4363$s instead. This is a breaking change that was not announced or mentioned anywhere.

The previous behavior was to treat slots with missing corresponding elements inside "with" as if they have an empty string. Instead, nothing is slotted in at all, not even the "Hello", and it just displays the raw string instead.

Only translations where there are corresponding elements inside "with" for all slots are unaffected, like this one:

/tellraw @a {"translate":"%s%1$s","with":["Hello"]}

(Displays HelloHello)

How to reproduce

1. Run the following command

   /tellraw @a {"translate":"%s%4363$s","with":["Hello"]}
   

2. Notice that %s%4363$s is printed to chat. Expected is that it would print Hello.

Code Analysis

This is caused by a change in the TranslatableContents class in 1.19.3-pre1. The decomposeTemplate method previously checked the replacement index against the length of "with" before calling getArgument and accepting the return value into the result.
Now getArgument is always called and throws an exception instead when it performs the same check. The exception gets caught within decompose, which then returns the raw translation string instead of the decomposed one.