If the author of the library is unable to reproduce the bug based upon your report, then it is unreasonable to expect him to spend a lot of time on it, let alone fix it.
But you also have a limited amount of time spend working on a product that is peripheral to your interest. Unfortunately, this may mean that the bug continues to exist, and no work is done on resolving it.
Fortunately this is not necessarily a disaster -- while in an ideal world, all software would be bug free, that isn't the case, and so we have to prioritize based upon the problems it causes US.
This means that it is indeed your responsibility to develop a reproducible test case IF YOU WANT IT FIXED. You may not care whether it gets fixed, and in that case, you have done everything that can and should be expected of you. You may want it fixed, but not enough to devote time to make it reproducible at this time. That is perfectly acceptable.
Reporting a bug to the best of your ability in the time you have to deal with it is simply good citizenship, you don't need to go beyond that unless it is necessary for your program. And you may not want to do so even then, there may be another library that you could use, or it may be possible to roll your own in a reasonable period of time. Basically it's up to you to decide what and what kind of effort it is worth to you.