Welcome!
For whatever reason it seems that in some cases the _id field is not provided. I'd double check the code because I suspect the problem is there.
May be add a logger to print whenever the computed id is empty, just before posting the request to Elasticsearch.
Without seeing your code, I can't really tell more...
BTW, 7.5 is totally outdated and has not been receiving the needed security patches.
At the very least, you should upgrade to 7.17.29. But better to switch to 8.19 or 9.2.0.