So it looks like Hebrew works differently now (see T167058), and it's probably good.

But this particular bug is still not resolved, probably because "טריפלקס" is a relatively rare loanword.

I'm wondering: Is there a way to add words to the dictionary?

For better or worse we discussed this a while back.

My summary after skimming that conversation is:

• automatic prefix-detection is certainly possible, but likely to cause errors, which is probably why HebMorph (the new plugin) doesn't do it.
• HebMorph isn't upgrading their open-source dictionary because they have upgraded their proprietary dictionary
• HebMorph uses an old version of the Hspell dictionary, so the newer one would probably be better, but it is compiled in a custom way for HebMorph, so it's non-trivial to do so.

My conclusion was:

Our options seem to be limited to forking and improving the dictionary independently, either by updating to Hspell v1.3, or manually adding new words. That is a bummer.

It might be possible to update and compile the dictionary independently of the plugin code, but that would make deployment more complex, since we'd have to deploy the code and dictionary updates separately. We've made updates just recently to make some complexity easier to handle—which is why enabling the new Hebrew plugin took several months—but I'm not sure how hard it would be to handle separate plugin/dictionary deployments. @dcausse might be able to say—but if it's possible to support, it would still require someone to take on the task of figuring out how to compile Hspell into a form HebMorph can use, and then maintaining it as needed.

Sure, it's certainly feasible. I currently don't know enough about Hebmorph to tell precisely how we could do.
Deploying lexical resources in addition to the hebmorph plugin is certainly possible. We could create a new static project called 'wmf-search-dictionnaries' and add all the lexical resources there.
This project would be deployed as a debian package on elastic machines.
If someone is willing to make a proof of concept that augmenting hebmorph with a dictionary can fix some issues I'd be happy to help on making it production ready and easy to deploy.

@Amire80 wondering if this is an issue on any of Hebrew MediaWiki websites, or only on Wikimedia? If the second, then this is a problem in cirrussearch rather than mwsearch.

The fix is only for CirrusSearch. Almost certainly though if you are using something like mysql search the same problem would exist (and many more, as stemming support in sql is exceptionally minimal).

Can this at least be marked as "stalled" or something? The difficulty in fixing is understandable, but the bug is real.

This seems to be blocked on upstream hence adding Upstream.
https://github.com/synhershko/HebMorph/issues lists a request to update to hspell 1.4 but no request to support ES6.