When vs If and asking in questions

“If” implies that you’re not sure whether you will see him again.

“When” implies that you’re not sure at what time or date you will see him again.

https://dictionary.cambridge.org/us/grammar/british-grammar/if-or-when

I think you've been told correctly. "I don't know when" doesn't inherently imply that you expect something to happen eventually.

Often context will make it clear. That's what happens in your second example of ending the date -- it would be considered inappropriate to say it that way if you don't expect a second date (although maybe the speaker is trying not to hurt the date's feelings by telling them outright that they don't want a second date).

In other cases you would usually try to make the conditions clearer:

I'd like to see you again the next time I'm in town, but I don't know when that will be.

Perhaps, where and when are physical terms, of space and time, and, if is a logical, or abstract, mental term that may be applied to either of where and when.

Another distinction may be made, which I hadn't thought of, before. Namely, that not specifying the when, means that the where is specified, or assumed, and, versa. The word, if, is again the general case, whereby both the when, and the where, of something are unspecified but tied into some logical sequence of events.