I have vague memories of also running into something where the a prop should not have possibly been undefined and yet it was, but as I recall (vaguely) we dealt with it by just using?. everywhere, or in your case{#if placeholders}{placeholders}{/if}. My best guess at the time, as yours is, was that it was a race condition somewhere, but I didn't have the time to invest in reproducing it since I couldn't trigger it reliably.

As for a repro, if it is a race condition my best suggestion would be to create a repro test case, either on the svelte.dev playground or on StackBlitz if SvelteKit is involved, similar to the pared-down example you put in this discussion. Then stick logs of console.log calls in there (yes, printf-style debugging is still all too necessary sometimes, sigh...) and write a script that will load the page repeatedly until it manages to trigger the bug. (With an appropriate pause between runs so you don't look like you're trying to DOS the Svelte playground or StackBlitz...) Then capture the logs from the one failed run.

Who knows how many runs you'll have to make before triggering the race condition, but if you can manage to trigger the race condition, that should be enough to qualify as a minimal repro for reporting the issue.