I have now edited the example I gave https://dreams.wikibase.cloud/w/index.php?title=Item:Q1149&diff=6989&oldid=6689 . That seemed to update the triple store.

Some work was done to make the query service updating more reliable in T284319
However there is still a condition where triples can be missed, so this is still best effort currently.
As you say, editing an entity again in 99.9% of cases will result in the missing triples appearing.

The edge case that is now handeled currently is:

• If the updater picks up a batch of changes to send to the backend
• And the backend doesnt respond because it is overloaded or down or restarting etc
• Then those triple updates will be lost

In code:

• Batches of entity changes are served from the platform API https://github.com/wbstack/api/blob/main/app/Http/Controllers/Backend/QsController.php#L12
• This is the API that the updater uses https://github.com/wbstack/queryservice-updater/blob/main/src/main/java/org/wikidata/query/rdf/tool/WbStackUpdate.java#L161
• When batches are returned by this API they are marked as done stragiht away before the updater does anything https://github.com/wbstack/api/blob/main/app/Http/Controllers/Backend/QsController.php#L92
• There is initial code for having the updater report back if batches are done or not https://github.com/wbstack/api/blob/main/app/Http/Controllers/Backend/QsController.php#L100-L122 But this would require move work in the Java updater code that is not completed yet
• This work would also be needed for further scaling the number of updaters and or backends that could be run

@Tarrow I have sampled a few items and could not immediately identify a problem.