Asking for an acknowledgement

On the other hand, it seems really awkward to demand that someone acknowledge me - it seems like a faux pas even to admit that I noticed.

Don't be embarrassed to ask to be credited for your work. If you're not willing to stand up for yourself as a professional, others may not do it for you!

An acknowledgment is entirely appropriate to recognize your contributions, which are technically useful, but not necessarily rising to the level where it is directly tied in to the goal of the final paper. To ask for it, I would ask the first author:

I think my bug fixes helped you carry out the research. Do you think you could mention my contribution in the acknowledgments of your new paper?

This makes it clear what you want and why you think it's useful. An acknowledgment is "free" in the sense that it doesn't dilute the author list, and there's no logical reason why you couldn't be accommodated.

Adding an acknowledgment essentially doesn't cost anything, so there's little reason why the authors wouldn't want to do it if you helped; however it is true that there is still some awkwardness involved in asking others to credit you.

One way to defuse the awkwardness could be to focus on tangible practical reasons why you think this acknowledgment could be useful to you, instead of more sensitive subjective reasons (e.g., I want to be acknowledged, I feel like I deserve the acknowledgment, etc.) E.g.,

In case my bug fixes were useful to you, do you think you could point it out in the acknowledgements section? This could be helpful later to show to [thesis committee / potential future advisor / supervisor / etc.] that I made some contribution to the paper.

Essentially the idea is to sidestep the embarrassment in "admitting that you noticed", or embarrassment on their part for not having thought about it themselves, having possibly hurt your feelings, etc.; and just frame the request as something motivated by a practical need. (Of course the need may be completely hypothetical -- it's just an excuse to avoid mentioning more subjective motivations.)

If I were in your place, I would not ask to be mentioned in acknowledgments, especially if you are planning to work with authors in the future.

Not all battles are equally important, think in the long run :)

p.s. if the authorship were discussed, I would say fight for it, but acknowledgments...I would not.